diff --git a/scripts/exporter/romm_exporter.py b/scripts/exporter/romm_exporter.py
index 700029bb..01cf11ec 100644
--- a/scripts/exporter/romm_exporter.py
+++ b/scripts/exporter/romm_exporter.py
@@ -29,6 +29,9 @@ SOURCE_URL = (
 class Exporter(BaseExporter):
     """Write RomM's known_bios_files.json, corrected."""
 
+    # (system, filename) keys the platform itself declares, set by render.
+    _platform_keys: frozenset[tuple[str, str]] = frozenset()
+
     @staticmethod
     def platform_name() -> str:
         return "romm"
@@ -61,16 +64,20 @@ class Exporter(BaseExporter):
         """
         return native_id in SLUG_MAP
 
-    @classmethod
-    def writable(cls, fe: NativeFile, require: str = "") -> bool:
+    def writable(self, fe: NativeFile, require: str = "") -> bool:
         """What RomM already ships stays; the conditions gate additions.
 
         An entry of theirs that could never verify is still theirs, and the
-        round trip is not the place to decide otherwise.
+        round trip is not the place to decide otherwise. An addition whose
+        slug:filename key the platform already holds cannot be written: the
+        fixture is a dict, and the second write replaced RomM's Dreamcast
+        boot.bin with fbneo's 480-byte file of the same name.
         """
         if fe.platform is not None:
             return True
-        return cls._verifiable(fe) and cls._known_platform(fe.native_system)
+        if (fe.native_system, fe.name) in self._platform_keys:
+            return False
+        return self._verifiable(fe) and self._known_platform(fe.native_system)
 
     def render(
         self,
@@ -80,6 +87,12 @@ class Exporter(BaseExporter):
         scraped: dict | None = None,
     ) -> dict[str, str]:
         output: OrderedDict[str, dict] = OrderedDict()
+        self._platform_keys = {
+            (system.native_id, fe.name)
+            for system in systems.values()
+            for fe in system.files
+            if fe.platform is not None
+        }
 
         for system in sorted(systems.values(), key=lambda s: s.native_id):
             for fe in sorted(system.files, key=lambda f: f.name):
diff --git a/tests/test_export_counts.py b/tests/test_export_counts.py
index 75792d1e..aabdca1f 100644
--- a/tests/test_export_counts.py
+++ b/tests/test_export_counts.py
@@ -183,6 +183,21 @@ class OneFileOneIdentity(unittest.TestCase):
         declared = next(f for f in systems["dc"].files if f.platform is not None)
         self.assertIsNone(declared.truth)
 
+class RommKeepsItsOwnKeys(unittest.TestCase):
+    def test_a_same_named_addition_does_not_replace_their_entry(self):
+        """fbneo's 480-byte boot.bin overwrote RomM's Dreamcast boot.bin."""
+        from exporter.romm_exporter import Exporter as Romm
+
+        theirs = NativeFile("boot.bin", "dc/boot.bin", "dc",
+                            platform={"size": 2097152, "md5": A})
+        ours = NativeFile("boot.bin", "fbneo/boot.bin", "dc",
+                          truth={"size": 480, "crc32": "f0774fc2"})
+        exporter = Romm()
+        produced = exporter.render({"dc": NativeSystem("dc", files=[theirs, ours])}, None, {})
+        written = json.loads(produced[exporter.native_filename()])
+        self.assertEqual(written["dc:boot.bin"]["size"], "2097152")
+        self.assertFalse(exporter.writable(ours))
+
 class RecalboxKeepsItsOwnNotes(unittest.TestCase):
     def test_no_profile_prose_reaches_a_note(self):
         fe = NativeFile("bios.bin", "bios.bin", "psx", platform={"md5": A},
diff --git a/tests/test_exporters.py b/tests/test_exporters.py
index 82a6c09f..50e4fcaf 100644
--- a/tests/test_exporters.py
+++ b/tests/test_exporters.py
@@ -206,7 +206,7 @@ class Reconciliation(unittest.TestCase):
                     if entry.platform is None:
                         continue
                     with self.subTest(platform=name, file=entry.name):
-                        self.assertTrue(cls.writable(entry))
+                        self.assertTrue(cls().writable(entry))
 
     def test_an_addition_the_format_cannot_express_is_refused(self):
         truth = {