hdl._ir: raise an error when an elaboratable is duplicated in hierarchy.

Fixes #1194.

amaranth/hdl/_ir.py

@@ -9,8 +9,8 @@ from . import _ast, _cd, _ir, _nir
 
 
 __all__ = [
-    "UnusedElaboratable", "Elaboratable", "DriverConflict", "Fragment", "Instance",
-    "IOBufferInstance", "PortDirection", "Design", "build_netlist",
+    "UnusedElaboratable", "Elaboratable", "DuplicateElaboratable", "DriverConflict", "Fragment",
+    "Instance", "IOBufferInstance", "PortDirection", "Design", "build_netlist",
 ]
 
 
@@ -30,6 +30,10 @@ class DriverConflict(UserWarning):
     pass
 
 
+class DuplicateElaboratable(Exception):
+    pass
+
+
 class Fragment:
     @staticmethod
     def get(obj, platform):
@@ -430,6 +434,7 @@ class Design:
         self.hierarchy = hierarchy
         self.fragments: dict[Fragment, DesignFragmentInfo] = {}
         self.signal_lca = _ast.SignalDict()
+        self.elaboratables: dict[Elaboratable, Fragment] = {}
         self._compute_fragment_depth_parent(fragment, None, 0)
         self._collect_used_signals(fragment)
         self._add_io_ports()
@@ -598,6 +603,15 @@ class Design:
         frag_info = self.fragments[fragment]
         frag_info.name = hierarchy
 
+        if fragment.origins is not None:
+            for origin in fragment.origins:
+                if origin in self.elaboratables:
+                    other_hierarchy = self.fragments[self.elaboratables[origin]].name
+                    raise DuplicateElaboratable(f"Elaboratable {origin!r} is included twice "
+                                                f"in the hierarchy, as {'.'.join(other_hierarchy)} "
+                                                f"and {'.'.join(hierarchy)}")
+                self.elaboratables[origin] = fragment
+
         if fragment is self.fragment:
             # Reserve names for top-level ports. If equal to the signal name, let the signal share it.
             for name, conn, _dir in self.ports:

amaranth/hdl/_xfrm.py

@@ -318,6 +318,7 @@ class FragmentTransformer:
         else:
             new_fragment = Fragment(src_loc=fragment.src_loc)
         new_fragment.attrs = OrderedDict(fragment.attrs)
+        new_fragment.origins = fragment.origins
         self.map_subfragments(fragment, new_fragment)
         self.map_domains(fragment, new_fragment)
         self.map_statements(fragment, new_fragment)

tests/test_hdl_ir.py

@@ -77,6 +77,18 @@ class FragmentDriversTestCase(FHDLTestCase):
         self.assertEqual(list(f.iter_sync()), [])
 
 
+class DuplicateElaboratableTestCase(FHDLTestCase):
+    def test_duplicate(self):
+        sub = Module()
+        m = Module()
+        m.submodules.a = sub
+        m.submodules.b = sub
+        with self.assertRaisesRegex(DuplicateElaboratable,
+                r"^Elaboratable .* is included twice in the hierarchy, as "
+                r"top\.a and top\.b$"):
+            Fragment.get(m, None).prepare()
+
+
 class FragmentPortsTestCase(FHDLTestCase):
     def setUp(self):
         self.s1 = Signal()