diff --git a/.gitignore b/.gitignore
index 2b3ab2c0b5d..1f063f79eff 100644
--- a/.gitignore
+++ b/.gitignore
@@ -18,6 +18,7 @@ target/
 **/.cache
 pom.xml.versionsBackup
 dependency-reduced-pom.xml
+.m2_repo/
 
 # GEdit generated stuff
 *.*~
diff --git a/core/queryparser/sparql/src/main/java/org/eclipse/rdf4j/query/parser/sparql/PrefixDeclProcessor.java b/core/queryparser/sparql/src/main/java/org/eclipse/rdf4j/query/parser/sparql/PrefixDeclProcessor.java
index 15612f35d5e..fa6352367be 100644
--- a/core/queryparser/sparql/src/main/java/org/eclipse/rdf4j/query/parser/sparql/PrefixDeclProcessor.java
+++ b/core/queryparser/sparql/src/main/java/org/eclipse/rdf4j/query/parser/sparql/PrefixDeclProcessor.java
@@ -69,10 +69,15 @@ public static Map<String, String> process(ASTOperationContainer qc, Map<String,
 			String iri = prefixDecl.getIRI().getValue();
 
 			if (prefixMap.containsKey(prefix)) {
-				throw new MalformedQueryException("Multiple prefix declarations for prefix '" + prefix + "'");
+				String existingIri = prefixMap.get(prefix);
+				if (!existingIri.equals(iri)) {
+					throw new MalformedQueryException("Multiple prefix declarations for prefix '" + prefix
+							+ "' with different namespaces: '" + existingIri + "' and '" + iri + "'");
+				}
+				// If the IRI is the same, allow the duplicate (no-op)
+			} else {
+				prefixMap.put(prefix, iri);
 			}
-
-			prefixMap.put(prefix, iri);
 		}
 
 		int preDefaultPrefixes = 0;
diff --git a/core/queryparser/sparql/src/test/java/org/eclipse/rdf4j/query/parser/sparql/SPARQLParserTest.java b/core/queryparser/sparql/src/test/java/org/eclipse/rdf4j/query/parser/sparql/SPARQLParserTest.java
index fa963633783..c0703660327 100644
--- a/core/queryparser/sparql/src/test/java/org/eclipse/rdf4j/query/parser/sparql/SPARQLParserTest.java
+++ b/core/queryparser/sparql/src/test/java/org/eclipse/rdf4j/query/parser/sparql/SPARQLParserTest.java
@@ -1047,4 +1047,59 @@ private Object bytesToObject(byte[] str) {
 			throw new RuntimeException(e);
 		}
 	}
+
+	@Test
+	public void testDuplicatePrefixDeclarations_SameNamespace_ShouldPass() {
+		// Test that duplicate prefix declarations with the same namespace are allowed
+		String query = "PREFIX foaf: <http://xmlns.com/foaf/0.1/>\n" +
+				"PREFIX foaf: <http://xmlns.com/foaf/0.1/>\n" +
+				"SELECT ?name WHERE { ?person foaf:name ?name }";
+
+		// This should not throw an exception
+		assertDoesNotThrow(() -> parser.parseQuery(query, null));
+
+		ParsedQuery parsed = parser.parseQuery(query, null);
+		assertNotNull(parsed);
+	}
+
+	@Test
+	public void testDuplicatePrefixDeclarations_DifferentNamespace_ShouldFail() {
+		// Test that duplicate prefix declarations with different namespaces are rejected
+		String query = "PREFIX foaf: <http://xmlns.com/foaf/0.1/>\n" +
+				"PREFIX foaf: <http://example.org/different/>\n" +
+				"SELECT ?name WHERE { ?person foaf:name ?name }";
+
+		// This should throw a MalformedQueryException
+		assertThatExceptionOfType(MalformedQueryException.class)
+				.isThrownBy(() -> parser.parseQuery(query, null))
+				.withMessageContaining("Multiple prefix declarations")
+				.withMessageContaining("foaf");
+	}
+
+	@Test
+	public void testDuplicatePrefixDeclarations_EmptyPrefix_SameNamespace_ShouldPass() {
+		// Test that duplicate default prefix declarations with the same namespace are allowed
+		String query = "PREFIX : <http://example.org/ns#>\n" +
+				"PREFIX : <http://example.org/ns#>\n" +
+				"SELECT ?name WHERE { :person :name ?name }";
+
+		// This should not throw an exception
+		assertDoesNotThrow(() -> parser.parseQuery(query, null));
+
+		ParsedQuery parsed = parser.parseQuery(query, null);
+		assertNotNull(parsed);
+	}
+
+	@Test
+	public void testDuplicatePrefixDeclarations_EmptyPrefix_DifferentNamespace_ShouldFail() {
+		// Test that duplicate default prefix declarations with different namespaces are rejected
+		String query = "PREFIX : <http://example.org/ns#>\n" +
+				"PREFIX : <http://example.org/different/>\n" +
+				"SELECT ?name WHERE { :person :name ?name }";
+
+		// This should throw a MalformedQueryException
+		assertThatExceptionOfType(MalformedQueryException.class)
+				.isThrownBy(() -> parser.parseQuery(query, null))
+				.withMessageContaining("Multiple prefix declarations");
+	}
 }
diff --git a/core/rio/turtle/src/test/java/org/eclipse/rdf4j/rio/turtle/TurtlePrefixDuplicateTest.java b/core/rio/turtle/src/test/java/org/eclipse/rdf4j/rio/turtle/TurtlePrefixDuplicateTest.java
new file mode 100644
index 00000000000..de87f336022
--- /dev/null
+++ b/core/rio/turtle/src/test/java/org/eclipse/rdf4j/rio/turtle/TurtlePrefixDuplicateTest.java
@@ -0,0 +1,288 @@
+/*******************************************************************************
+ * Copyright (c) 2025 Eclipse RDF4J contributors.
+ *
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Distribution License v1.0
+ * which accompanies this distribution, and is available at
+ * http://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ *******************************************************************************/
+package org.eclipse.rdf4j.rio.turtle;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.io.StringReader;
+import java.util.ArrayList;
+import java.util.List;
+
+import org.eclipse.rdf4j.model.Statement;
+import org.eclipse.rdf4j.rio.RDFHandler;
+import org.eclipse.rdf4j.rio.RDFParseException;
+import org.eclipse.rdf4j.rio.RDFParser;
+import org.eclipse.rdf4j.rio.helpers.StatementCollector;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Tests for duplicate prefix declarations in Turtle parser. Tests include scenarios with @base to show how different
+ * placement of the base changes duplicate relative prefix declarations.
+ */
+public class TurtlePrefixDuplicateTest {
+
+	private RDFParser parser;
+	private List<Statement> statements;
+	private RDFHandler handler;
+
+	@BeforeEach
+	public void setUp() {
+		parser = new TurtleParser();
+		statements = new ArrayList<>();
+		handler = new StatementCollector(statements);
+		parser.setRDFHandler(handler);
+	}
+
+	@Test
+	public void testDuplicatePrefixDeclarations_SameNamespace_ShouldPass() throws Exception {
+		String turtle = "@prefix foaf: <http://xmlns.com/foaf/0.1/> .\n" +
+				"@prefix foaf: <http://xmlns.com/foaf/0.1/> .\n" +
+				"\n" +
+				"<http://example.org/person> foaf:name \"John Doe\" .\n";
+
+		// Should not throw an exception when same prefix maps to same namespace
+		assertDoesNotThrow(() -> parser.parse(new StringReader(turtle), "http://example.org/"));
+
+		// Should produce the expected statement
+		assertThat(statements).hasSize(1);
+		assertThat(statements.get(0).getPredicate().toString()).isEqualTo("http://xmlns.com/foaf/0.1/name");
+	}
+
+	@Test
+	public void testDuplicatePrefixDeclarations_DifferentNamespace_LastOneWins() throws Exception {
+		String turtle = "@prefix foaf: <http://xmlns.com/foaf/0.1/> .\n" +
+				"@prefix foaf: <http://example.org/different/> .\n" +
+				"\n" +
+				"<http://example.org/person> foaf:name \"John Doe\" .\n";
+
+		// Should not throw an exception - Turtle parsers typically allow redefinition
+		assertDoesNotThrow(() -> parser.parse(new StringReader(turtle), "http://example.org/"));
+
+		// The last prefix declaration should win
+		assertThat(statements).hasSize(1);
+		assertThat(statements.get(0).getPredicate().toString()).isEqualTo("http://example.org/different/name");
+	}
+
+	@Test
+	public void testDuplicateDefaultPrefixDeclarations_SameNamespace_ShouldPass() throws Exception {
+		String turtle = "@prefix : <http://example.org/ns#> .\n" +
+				"@prefix : <http://example.org/ns#> .\n" +
+				"\n" +
+				"<http://example.org/person> :name \"John Doe\" .\n";
+
+		// Should not throw an exception when same default prefix maps to same namespace
+		assertDoesNotThrow(() -> parser.parse(new StringReader(turtle), "http://example.org/"));
+
+		// Should produce the expected statement
+		assertThat(statements).hasSize(1);
+		assertThat(statements.get(0).getPredicate().toString()).isEqualTo("http://example.org/ns#name");
+	}
+
+	// Tests with @base showing different scenarios with relative prefix declarations
+
+	@Test
+	public void testDuplicateRelativePrefix_SameBase_ShouldPass() throws Exception {
+		String turtle = "@base <http://example.org/> .\n" +
+				"@prefix rel: <vocab/> .\n" +
+				"@prefix rel: <vocab/> .\n" +
+				"\n" +
+				"<http://example.org/person> rel:name \"Alice\" .\n";
+
+		// Should not throw an exception when same relative prefix resolves to same absolute namespace
+		assertDoesNotThrow(() -> parser.parse(new StringReader(turtle), "http://example.org/"));
+
+		// Should produce the expected statement with resolved absolute namespace
+		assertThat(statements).hasSize(1);
+		assertThat(statements.get(0).getPredicate().toString()).isEqualTo("http://example.org/vocab/name");
+	}
+
+	@Test
+	public void testDuplicateRelativePrefix_BaseChangedBetween_LastWins() throws Exception {
+		String turtle = "@base <http://example.org/> .\n" +
+				"@prefix rel: <vocab/> .\n" +
+				"@base <http://different.org/> .\n" +
+				"@prefix rel: <vocab/> .\n" +
+				"\n" +
+				"<http://example.org/person> rel:name \"Bob\" .\n";
+
+		// Should not throw exception - turtle allows redefinition, last one wins
+		assertDoesNotThrow(() -> parser.parse(new StringReader(turtle), "http://example.org/"));
+
+		// The last prefix declaration should win (resolved with the second base)
+		assertThat(statements).hasSize(1);
+		assertThat(statements.get(0).getPredicate().toString()).isEqualTo("http://different.org/vocab/name");
+	}
+
+	@Test
+	public void testDuplicateRelativePrefix_ExternalBaseChange_LastWins() throws Exception {
+		String turtle = "@prefix rel: <vocab/> .\n" +
+				"@base <http://different.org/> .\n" +
+				"@prefix rel: <vocab/> .\n" +
+				"\n" +
+				"<http://example.org/person> rel:name \"Charlie\" .\n";
+
+		// Should not throw exception - turtle allows redefinition
+		assertDoesNotThrow(() -> parser.parse(new StringReader(turtle), "http://example.org/"));
+
+		// First prefix uses external base, second uses internal base
+		assertThat(statements).hasSize(1);
+		assertThat(statements.get(0).getPredicate().toString()).isEqualTo("http://different.org/vocab/name");
+	}
+
+	@Test
+	public void testDuplicateRelativePrefix_MultipleBaseChanges_LastWins() throws Exception {
+		String turtle = "@base <http://first.org/> .\n" +
+				"@prefix rel: <vocab/> .\n" +
+				"@base <http://second.org/> .\n" +
+				"@base <http://third.org/> .\n" +
+				"@prefix rel: <vocab/> .\n" +
+				"\n" +
+				"<http://example.org/person> rel:name \"David\" .\n";
+
+		// Should not throw exception
+		assertDoesNotThrow(() -> parser.parse(new StringReader(turtle), "http://example.org/"));
+
+		// Last prefix with last base should win
+		assertThat(statements).hasSize(1);
+		assertThat(statements.get(0).getPredicate().toString()).isEqualTo("http://third.org/vocab/name");
+	}
+
+	@Test
+	public void testDuplicateRelativePrefix_BaseAfterAllPrefixes_EarlierBasesUsed() throws Exception {
+		String turtle = "@base <http://first.org/> .\n" +
+				"@prefix rel: <vocab/> .\n" +
+				"@base <http://second.org/> .\n" +
+				"@prefix rel: <vocab/> .\n" +
+				"@base <http://third.org/> .\n" +
+				"\n" +
+				"<http://example.org/person> rel:name \"Eve\" .\n";
+
+		// Should not throw exception
+		assertDoesNotThrow(() -> parser.parse(new StringReader(turtle), "http://example.org/"));
+
+		// Second prefix with second base should be used (base after prefixes doesn't affect them)
+		assertThat(statements).hasSize(1);
+		assertThat(statements.get(0).getPredicate().toString()).isEqualTo("http://second.org/vocab/name");
+	}
+
+	@Test
+	public void testDuplicateRelativePrefix_SameExternalBase_ShouldPass() throws Exception {
+		String turtle = "@prefix rel: <vocab/> .\n" +
+				"@prefix rel: <vocab/> .\n" +
+				"\n" +
+				"<http://example.org/person> rel:name \"Frank\" .\n";
+
+		// Should not throw exception when both relative prefixes resolve to same namespace using external base
+		assertDoesNotThrow(() -> parser.parse(new StringReader(turtle), "http://example.org/"));
+
+		// Both prefixes resolve to same namespace using external base
+		assertThat(statements).hasSize(1);
+		assertThat(statements.get(0).getPredicate().toString()).isEqualTo("http://example.org/vocab/name");
+	}
+
+	@Test
+	public void testDuplicateRelativePrefix_SameInternalBase_ShouldPass() throws Exception {
+		String turtle = "@base <http://example.org/ns/> .\n" +
+				"@prefix rel: <vocab/> .\n" +
+				"@prefix rel: <vocab/> .\n" +
+				"\n" +
+				"<http://example.org/person> rel:name \"Grace\" .\n";
+
+		// Should not throw exception when both relative prefixes resolve to same namespace
+		assertDoesNotThrow(() -> parser.parse(new StringReader(turtle), "http://example.org/"));
+
+		// Both prefixes resolve to same namespace using internal base
+		assertThat(statements).hasSize(1);
+		assertThat(statements.get(0).getPredicate().toString()).isEqualTo("http://example.org/ns/vocab/name");
+	}
+
+	@Test
+	public void testDuplicateRelativePrefix_AbsoluteToRelative_LastWins() throws Exception {
+		String turtle = "@prefix rel: <http://absolute.org/vocab/> .\n" +
+				"@base <http://relative.org/> .\n" +
+				"@prefix rel: <vocab/> .\n" +
+				"\n" +
+				"<http://example.org/person> rel:name \"Henry\" .\n";
+
+		// Should not throw exception
+		assertDoesNotThrow(() -> parser.parse(new StringReader(turtle), "http://example.org/"));
+
+		// Second (relative) prefix should win
+		assertThat(statements).hasSize(1);
+		assertThat(statements.get(0).getPredicate().toString()).isEqualTo("http://relative.org/vocab/name");
+	}
+
+	@Test
+	public void testDuplicateRelativePrefix_RelativeToAbsolute_LastWins() throws Exception {
+		String turtle = "@base <http://relative.org/> .\n" +
+				"@prefix rel: <vocab/> .\n" +
+				"@prefix rel: <http://absolute.org/vocab/> .\n" +
+				"\n" +
+				"<http://example.org/person> rel:name \"Ivy\" .\n";
+
+		// Should not throw exception
+		assertDoesNotThrow(() -> parser.parse(new StringReader(turtle), "http://example.org/"));
+
+		// Second (absolute) prefix should win
+		assertThat(statements).hasSize(1);
+		assertThat(statements.get(0).getPredicate().toString()).isEqualTo("http://absolute.org/vocab/name");
+	}
+
+	@Test
+	public void testDuplicateRelativePrefix_ComplexRelativePaths_LastWins() throws Exception {
+		String turtle = "@base <http://example.org/path/> .\n" +
+				"@prefix rel: <../vocab/> .\n" +
+				"@base <http://example.org/different/path/> .\n" +
+				"@prefix rel: <../../vocab/> .\n" +
+				"\n" +
+				"<http://example.org/person> rel:name \"Jack\" .\n";
+
+		// Should not throw exception
+		assertDoesNotThrow(() -> parser.parse(new StringReader(turtle), "http://example.org/"));
+
+		// Last prefix with complex relative path should win
+		assertThat(statements).hasSize(1);
+		assertThat(statements.get(0).getPredicate().toString()).isEqualTo("http://example.org/vocab/name");
+	}
+
+	@Test
+	public void testDuplicateRelativePrefix_NoBaseForRelative_ShouldFail() throws Exception {
+		String turtle = "@prefix rel: <vocab/> .\n" +
+				"@prefix rel: <vocab/> .\n" +
+				"\n" +
+				"<http://example.org/person> rel:name \"Kate\" .\n";
+
+		// Should throw exception when relative prefix cannot be resolved (no base provided)
+		assertThrows(RDFParseException.class, () -> {
+			parser.parse(new StringReader(turtle), null);
+		});
+	}
+
+	@Test
+	public void testDuplicateDefaultRelativePrefix_BaseChanges_LastWins() throws Exception {
+		String turtle = "@base <http://first.org/> .\n" +
+				"@prefix : <vocab/> .\n" +
+				"@base <http://second.org/> .\n" +
+				"@prefix : <vocab/> .\n" +
+				"\n" +
+				"<http://example.org/person> :name \"Luna\" .\n";
+
+		// Should not throw exception
+		assertDoesNotThrow(() -> parser.parse(new StringReader(turtle), "http://example.org/"));
+
+		// Last default prefix declaration should win
+		assertThat(statements).hasSize(1);
+		assertThat(statements.get(0).getPredicate().toString()).isEqualTo("http://second.org/vocab/name");
+	}
+}
\ No newline at end of file