hdl._ir: Handle duplicate fragment names.

amaranth/hdl/_ir.py

@@ -581,13 +581,14 @@ class Design:
         """
         self.fragment_names[fragment] = hierarchy
 
-        signal_names = set(self.signal_names[fragment].values())
+        taken_names = set(self.signal_names[fragment].values())
         for subfragment_index, (subfragment, subfragment_name, subfragment_src_loc) in enumerate(fragment.subfragments):
             if subfragment_name is None:
                 subfragment_name = f"U${subfragment_index}"
-            elif subfragment_name in signal_names:
+            elif subfragment_name in taken_names:
                 subfragment_name = f"{subfragment_name}$U${subfragment_index}"
-            assert subfragment_name not in signal_names
+            assert subfragment_name not in taken_names
+            taken_names.add(subfragment_name)
             self._assign_names_to_fragments(subfragment, hierarchy=(*hierarchy, subfragment_name))
 
 

tests/test_hdl_ir.py

@@ -1030,6 +1030,18 @@ class NamesTestCase(FHDLTestCase):
             a_f: ("top", "a$U$0")
         })
 
+    def test_assign_names_to_fragments_duplicate(self):
+        f = Fragment()
+        f.add_subfragment(a1_f := Fragment(), name="a")
+        f.add_subfragment(a2_f := Fragment(), name="a")
+
+        design = Design(f, ports=[], hierarchy=("top",))
+        self.assertEqual(design.fragment_names, {
+            f: ("top",),
+            a1_f: ("top", "a"),
+            a2_f: ("top", "a$U$1"),
+        })
+
 
 class ElaboratesTo(Elaboratable):
     def __init__(self, lower):