Update.

src/fmripost_aroma/tests/test_utils_bids.py

@@ -346,21 +346,19 @@ def test_collect_derivatives_xsectional_02(tmpdir):
     check_expected(subject_data, expected)
 
     # Query for session 3 (no anat available) should raise an error
-    subject_data = xbids.collect_derivatives(
-        raw_dataset=None,
-        derivatives_dataset=layout,
-        entities={'subject': '102', 'session': '3'},
-        fieldmap_id=None,
-        spec=None,
+    with pytest.raises(
+        ValueError,
+        match='Multiple anatomical derivatives found for anat_mni152nlin6asym',
+    ):
+        subject_data = xbids.collect_derivatives(
+            raw_dataset=None,
+            derivatives_dataset=layout,
+            entities={'subject': '102', 'session': '3'},
+            fieldmap_id=None,
+            spec=None,
         patterns=None,
         allow_multiple=False,
     )
-    expected = {
-        'anat_mni152nlin6asym': (
-            'sub-102_ses-2_space-MNI152NLin6Asym_res-02_desc-preproc_T1w.nii.gz'
-        ),
-    }
-    check_expected(subject_data, expected)
 
 
 def test_collect_derivatives_xsectional_03(tmpdir):

src/fmripost_aroma/utils/bids.py

@@ -136,6 +136,11 @@ def collect_derivatives(
             if not item:
                 derivs_cache[k] = None
             elif not allow_multiple and len(item) > 1 and k.startswith('anat'):
+                # Raise an error if multiple derivatives are found from different sessions
+                item_sessions = [layout.get_file(f).entities['session'] for f in item]
+                if len(set(item_sessions)) > 1:
+                    raise ValueError(f'Multiple anatomical derivatives found for {k}: {item}')
+
                 # Anatomical derivatives are allowed to have multiple files (e.g., T1w and T2w)
                 # but we just grab the first one
                 derivs_cache[k] = item[0]