diff --git a/sdcflows/tests/test_fieldmaps.py b/sdcflows/tests/test_fieldmaps.py
index 383376b988..9ecef796df 100644
--- a/sdcflows/tests/test_fieldmaps.py
+++ b/sdcflows/tests/test_fieldmaps.py
@@ -120,7 +120,7 @@ def test_FieldmapEstimation(dsA_dir, inputfiles, method, nsources):
         fm.FieldmapEstimation(sources, bids_id=fe.bids_id)
 
     # Ensure we can't instantiate one more estimation with same sources
-    with pytest.raises(ValueError):
+    with pytest.warns(UserWarning, match=r'is already .* in mapping'):
         fm.FieldmapEstimation(sources, bids_id=f'my{fe.bids_id}')
 
     # Exercise workflow creation
diff --git a/sdcflows/utils/bimap.py b/sdcflows/utils/bimap.py
index bb360daea3..a907b6745d 100644
--- a/sdcflows/utils/bimap.py
+++ b/sdcflows/utils/bimap.py
@@ -23,6 +23,7 @@
 """A bidirectional hashmap."""
 
 import re
+import warnings
 
 _autokey_pat = re.compile(r'^auto_(\d+)$')
 
@@ -144,8 +145,10 @@ def __setitem__(self, key, value):
         if self.__contains__(key):
             raise KeyError(f"'{key}' is already {'a value' * (key in self._inverse)} in mapping")
         if self.__contains__(value):
-            raise ValueError(
-                f"'{value}' is already {'a key' * (value not in self._inverse)} in mapping"
+            warnings.warn(
+                f"'{value}' is already {'a key' * (value not in self._inverse)} in mapping",
+                UserWarning,
+                stacklevel=2,
             )
 
         super().__setitem__(key, value)