fix: write one romm entry per key

scripts/exporter/romm_exporter.py

@@ -29,8 +29,12 @@ SOURCE_URL = (
 class Exporter(BaseExporter):
     """Write RomM's known_bios_files.json, corrected."""
 
-    # (system, filename) keys the platform itself declares, set by render.
+    def __init__(self) -> None:
-    _platform_keys: frozenset[tuple[str, str]] = frozenset()
+        super().__init__()
+        # (system, filename) keys the platform itself declares, and the
+        # addition that claims each other key first: both set by render.
+        self._platform_keys: set[tuple[str, str]] = set()
+        self._first_addition: dict[tuple[str, str], int] = {}
 
     @staticmethod
     def platform_name() -> str:
@@ -75,9 +79,15 @@ class Exporter(BaseExporter):
         """
         if fe.platform is not None:
             return True
-        if (fe.native_system, fe.name) in self._platform_keys:
+        key = (fe.native_system, fe.name)
+        if key in self._platform_keys:
             return False
-        return self._verifiable(fe) and self._known_platform(fe.native_system)
+        if not (self._verifiable(fe) and self._known_platform(fe.native_system)):
+            return False
+        # Two additions under one key: the second would overwrite the first
+        # in the dict and both were counted landed. The first one in render
+        # order is written, the other is not.
+        return self._first_addition.get(key, id(fe)) == id(fe)
 
     def render(
         self,
@@ -93,6 +103,17 @@ class Exporter(BaseExporter):
             for fe in system.files
             if fe.platform is not None
         }
+        self._first_addition = {}
+        for system in sorted(systems.values(), key=lambda s: s.native_id):
+            for fe in sorted(system.files, key=lambda f: f.name):
+                key = (fe.native_system, fe.name)
+                if (
+                    fe.platform is None
+                    and key not in self._platform_keys
+                    and self._verifiable(fe)
+                    and self._known_platform(fe.native_system)
+                ):
+                    self._first_addition.setdefault(key, id(fe))
 
         for system in sorted(systems.values(), key=lambda s: s.native_id):
             for fe in sorted(system.files, key=lambda f: f.name):

tests/test_export_counts.py

@@ -362,6 +362,28 @@ class RecalboxKeepsItsOwnNotes(unittest.TestCase):
                         truth={"md5": A, "note": "Loaded at libretro.c:120"})
         self.assertNotIn("note=", Recalbox()._bios_element(fe, "psx"))
 
+class RommWritesOneEntryPerKey(unittest.TestCase):
+    """Two additions under one slug:filename key: the second overwrote the
+    first in RomM's dict and both were counted landed."""
+
+    def test_the_second_addition_under_a_key_is_not_writable(self):
+        from exporter.romm_exporter import Exporter as Romm  # noqa: PLC0415
+
+        first = NativeFile("basic.rom", "Acorn/basic.rom", "acpc",
+                           truth={"size": 16384, "md5": A})
+        second = NativeFile("basic.rom", "Electron/basic.rom", "acpc",
+                            truth={"size": 16384, "md5": B})
+        exporter = Romm()
+        produced = exporter.render(
+            {"acpc": NativeSystem("acpc", files=[first, second])}, None, {}, None
+        )
+        data = json.loads(produced[exporter.native_filename()])
+        self.assertEqual(list(data), ["acpc:basic.rom"])
+        self.assertEqual(
+            [exporter.writable(first), exporter.writable(second)], [True, False]
+        )
+
+
 class RetroPieProposals(unittest.TestCase):
     def test_required_is_read_for_the_package_core(self):
         fe = NativeFile("scph5501.bin", "scph5501.bin", "psx",