diff --git a/src/core/duplicates.ts b/src/core/duplicates.ts
new file mode 100644
index 0000000..bc7fe89
--- /dev/null
+++ b/src/core/duplicates.ts
@@ -0,0 +1,175 @@
+/**
+ * Duplicate dependency detection logic.
+ *
+ * This module provides pure functions for detecting duplicate dependencies
+ * from lockfile data, without any formatting or presentation logic.
+ */
+
+import type {ParsedLockFile} from 'lockparse';
+
+/**
+ * A map of package names to their installed versions.
+ */
+export type VersionsMap = Map<string, Set<string>>;
+
+/**
+ * Information about a duplicate dependency.
+ */
+export interface DuplicateInfo {
+  /** The package name */
+  name: string;
+  /** Set of versions installed for this package */
+  versions: Set<string>;
+  /** Number of versions installed */
+  versionCount: number;
+}
+
+/**
+ * Summary of duplicate dependencies.
+ */
+export interface DuplicateSummary {
+  /** Total number of packages with duplicates */
+  duplicateCount: number;
+  /** List of duplicate packages */
+  duplicates: DuplicateInfo[];
+}
+
+/**
+ * Computes a map of package names to their installed versions from a lockfile.
+ *
+ * This extracts all unique package name + version combinations from the
+ * parsed lockfile.
+ *
+ * @param lockFile - Parsed lockfile from lockparse
+ * @returns Map of package names to sets of installed versions
+ */
+export function computeDependencyVersions(
+  lockFile: ParsedLockFile
+): VersionsMap {
+  const result: VersionsMap = new Map();
+
+  for (const pkg of lockFile.packages) {
+    if (!pkg.name || !pkg.version) continue;
+    addVersion(result, pkg.name, pkg.version);
+  }
+
+  return result;
+}
+
+/**
+ * Adds a version to the versions map.
+ *
+ * @param map - The versions map to update
+ * @param name - Package name
+ * @param version - Version string
+ */
+function addVersion(map: VersionsMap, name: string, version: string): void {
+  let set = map.get(name);
+  if (!set) {
+    set = new Set();
+    map.set(name, set);
+  }
+  set.add(version);
+}
+
+/**
+ * Detects duplicate dependencies from a versions map.
+ *
+ * A duplicate is defined as a package that has more versions installed
+ * than the specified threshold.
+ *
+ * @param versionMap - Map of package names to installed versions
+ * @param threshold - Minimum number of versions to consider a duplicate (default: 1)
+ * @returns Array of duplicate dependency info
+ */
+export function detectDuplicates(
+  versionMap: VersionsMap,
+  threshold: number = 1
+): DuplicateInfo[] {
+  const duplicates: DuplicateInfo[] = [];
+
+  for (const [name, versions] of versionMap) {
+    if (versions.size > threshold) {
+      duplicates.push({
+        name,
+        versions,
+        versionCount: versions.size
+      });
+    }
+  }
+
+  // Sort by version count descending, then by name
+  duplicates.sort((a, b) => {
+    if (b.versionCount !== a.versionCount) {
+      return b.versionCount - a.versionCount;
+    }
+    return a.name.localeCompare(b.name);
+  });
+
+  return duplicates;
+}
+
+/**
+ * Computes a summary of duplicate dependencies from a lockfile.
+ *
+ * This is a convenience function that combines computeDependencyVersions
+ * and detectDuplicates.
+ *
+ * @param lockFile - Parsed lockfile from lockparse
+ * @param threshold - Minimum number of versions to consider a duplicate (default: 1)
+ * @returns Summary of duplicate dependencies
+ */
+export function computeDuplicateSummary(
+  lockFile: ParsedLockFile,
+  threshold: number = 1
+): DuplicateSummary {
+  const versionMap = computeDependencyVersions(lockFile);
+  const duplicates = detectDuplicates(versionMap, threshold);
+
+  return {
+    duplicateCount: duplicates.length,
+    duplicates
+  };
+}
+
+/**
+ * Computes the diff between two version sets.
+ *
+ * This is useful for the GitHub Action to compare before/after states.
+ *
+ * @param prev - Previous versions map
+ * @param curr - Current versions map
+ * @returns Array of changes with package name, previous versions, and current versions
+ */
+export function diffDependencyVersions(
+  prev: VersionsMap,
+  curr: VersionsMap
+): Array<{name: string; previous: Set<string>; current: Set<string>}> {
+  const names = new Set<string>([...prev.keys(), ...curr.keys()]);
+  const changes: Array<{
+    name: string;
+    previous: Set<string>;
+    current: Set<string>;
+  }> = [];
+
+  for (const name of names) {
+    const a = prev.get(name) || new Set<string>();
+    const b = curr.get(name) || new Set<string>();
+    if (!setsEqual(a, b)) {
+      changes.push({name, previous: a, current: b});
+    }
+  }
+
+  return changes;
+}
+
+/**
+ * Checks if two sets are equal.
+ */
+function setsEqual(a: Set<string>, b: Set<string>): boolean {
+  if (a.size !== b.size) return false;
+  for (const v of a) {
+    if (!b.has(v)) return false;
+  }
+  return true;
+}
diff --git a/src/core/index.ts b/src/core/index.ts
new file mode 100644
index 0000000..730df62
--- /dev/null
+++ b/src/core/index.ts
@@ -0,0 +1,5 @@
+/**
+ * Core modules containing pure, reusable logic that can be imported
+ * by external tools like the e18e GitHub Action.
+ */
+export * from './duplicates.js';