diff --git a/scripts/exporter/base_exporter.py b/scripts/exporter/base_exporter.py
index d3f37166..8e022454 100644
--- a/scripts/exporter/base_exporter.py
+++ b/scripts/exporter/base_exporter.py
@@ -16,6 +16,10 @@ from .baseline import NativeFile, NativeSystem, Report
 class BaseExporter(ABC):
     """Base class for writing a platform's own BIOS file, corrected."""
 
+    # A hash field the checker compares to one value: an addition the
+    # truth accepts under several cannot be written whole.
+    SINGLE_HASH_FIELD: str | None = None
+
     @staticmethod
     @abstractmethod
     def platform_name() -> str:
@@ -145,6 +149,11 @@ class BaseExporter(ABC):
             return True
         if not cls.can_add() or (cls.keys_by_system() and fe.foreign_system):
             return False
+        single = cls.SINGLE_HASH_FIELD
+        if single and len(set(fe.hashes(single))) > 1:
+            # One value checked: written with the first, the addition
+            # refuses every other dump the core accepts.
+            return False
         require = require or cls.requires()
         return not require or bool(fe.hash(require))
 
diff --git a/scripts/exporter/batocera_exporter.py b/scripts/exporter/batocera_exporter.py
index 16b22292..0ebc6481 100644
--- a/scripts/exporter/batocera_exporter.py
+++ b/scripts/exporter/batocera_exporter.py
@@ -25,6 +25,11 @@ _INDENT = " " * 4
 class Exporter(BaseExporter):
     """Write Batocera's batocera-systems, corrected."""
 
+    # checkBios compares one md5 for a plain file; altmd5 is read only for
+    # a zippedFile: retrodream's boot.bin written with the first of four
+    # md5 refused the three other dumps the core accepts.
+    SINGLE_HASH_FIELD = "md5"
+
     # checkBios reads emulator names from es_systems, and Batocera's
     # standalone_cores are those names. ROCKNIX's list holds package names
     # (duckstation-sa), which its checker does not know.
@@ -58,14 +63,7 @@ class Exporter(BaseExporter):
 
     @classmethod
     def writable(cls, fe: NativeFile, require: str = "") -> bool:
-        """An addition the format can state whole.
-
-        checkBios compares one md5 for a plain file (altmd5 is read only for
-        a zippedFile): written with the first of four, retrodream's boot.bin
-        refused the three other dumps the core accepts.
-        """
-        if fe.platform is None and len(set(fe.hashes("md5"))) > 1:
-            return False
+        """An addition the format can state whole."""
         if fe.platform is None and not cls._usable_runners(fe):
             # Written without its core's emulator and core, checkBios asks
             # every build for it (the defect _runner_keys repairs for the
diff --git a/scripts/exporter/retrobat_exporter.py b/scripts/exporter/retrobat_exporter.py
index 48f74280..5f5b96ed 100644
--- a/scripts/exporter/retrobat_exporter.py
+++ b/scripts/exporter/retrobat_exporter.py
@@ -21,6 +21,9 @@ SOURCE_URL = (
 class Exporter(BaseExporter):
     """Write RetroBat's batocera-systems.json, corrected."""
 
+    # One md5 per entry, compared as Batocera's checkBios does.
+    SINGLE_HASH_FIELD = "md5"
+
     @staticmethod
     def platform_name() -> str:
         return "retrobat"
diff --git a/tests/test_export_counts.py b/tests/test_export_counts.py
index 9e435b60..7e94a6b3 100644
--- a/tests/test_export_counts.py
+++ b/tests/test_export_counts.py
@@ -336,6 +336,16 @@ class OneMd5FormatsAddWhole(unittest.TestCase):
         self.assertFalse(Batocera.writable(several))
         self.assertTrue(Batocera.writable(single))
 
+    def test_retrobat_refuses_it_too(self):
+        # RetroBat's batocera-systems.json holds one md5 per entry: the
+        # addition was written with the first and counted added.
+        from exporter.retrobat_exporter import Exporter as RetroBat  # noqa: PLC0415
+
+        several = NativeFile("boot.bin", "boot.bin", "dc", truth={"md5": [A, B, C]})
+        single = NativeFile("boot2.bin", "boot2.bin", "dc", truth={"md5": A})
+        self.assertFalse(RetroBat.writable(several))
+        self.assertTrue(RetroBat.writable(single))
+
 
 class BizHawkKeepsItsOwnHashes(unittest.TestCase):
     def test_a_declared_sha1_is_not_replaced(self):