diff --git a/install.ps1 b/install.ps1
index f1b5e2dc..3ecdc55f 100644
--- a/install.ps1
+++ b/install.ps1
@@ -8,7 +8,7 @@ param(
 
 $ErrorActionPreference = "Stop"
 $defaultInstallUrl = "https://raw.githubusercontent.com/Abdess/retrobios/main/install.py"
-$defaultInstallSha256 = "b163e90cc3101ce499e1f01899a398fbea27656054fb2fb9b31f07631d15f19a"
+$defaultInstallSha256 = "133242b850a1e2dfa820f39b466c8b56a9363aee1cc8df1e9a1f9f28744a0cdc"
 $maximumInstallerBytes = 2MB
 $installerExit = 0
 $installer = if ($PSScriptRoot) { Join-Path $PSScriptRoot "install.py" } else { $null }
diff --git a/install.py b/install.py
index 7da9e794..8a4f3461 100644
--- a/install.py
+++ b/install.py
@@ -1134,6 +1134,24 @@ def keeps_case(directory: Path) -> bool:
         os.unlink(name)
 
 
+def fold_case_twins(files: list[dict]) -> tuple[list[dict], list[str]]:
+    """One file per name where the directory folds case; the others named.
+
+    The first in manifest order stays, as the pack's own order places it.
+    """
+    kept: list[dict] = []
+    seen: set[str] = set()
+    folded: list[str] = []
+    for entry in files:
+        key = str(entry.get("dest", "")).casefold()
+        if key in seen:
+            folded.append(str(entry.get("dest", "")))
+            continue
+        seen.add(key)
+        kept.append(entry)
+    return kept, folded
+
+
 def installed_cores(dirs: list[Path]) -> list[str]:
     """The cores present in these directories, by the name the buildbot uses.
 
@@ -3218,10 +3236,22 @@ def _install(args: argparse.Namespace, real_stdout: TextIO) -> None:
         files = manifest.get("files", [])
         omitted_files = manifest.get("omitted_files", [])
         variants = manifest.get("case_variants", [])
-        if variants and keeps_case(bios_path):
+        case_kept = keeps_case(bios_path)
+        if variants and case_kept:
             # The list is folded for Windows and macOS; here bios.col and
             # BIOS.col are two files that two cores open.
             files = files + variants
+        if not case_kept:
+            files, folded = fold_case_twins(files)
+            if folded:
+                # Batocera's share opened over SMB, an exFAT card: the second
+                # spelling overwrote the first, and every --check found one
+                # of them wrong and fetched it again.
+                shown = ", ".join(folded[:3]) + (" ..." if len(folded) > 3 else "")
+                print(
+                    f"  {_plural(len(folded), 'file')} left out: {bios_path} does not keep "
+                    f"case and holds another file under the same name ({shown})"
+                )
 
         if args.list_targets:
             targets = fetch_targets(plat_name)
diff --git a/install.sh b/install.sh
index 8b8fac2f..810ce9a7 100755
--- a/install.sh
+++ b/install.sh
@@ -18,7 +18,7 @@ esac
 TEMP_INSTALLER=""
 TEMP_DIRECTORY=""
 DEFAULT_INSTALL_URL="https://raw.githubusercontent.com/Abdess/retrobios/main/install.py"
-DEFAULT_INSTALL_SHA256="b163e90cc3101ce499e1f01899a398fbea27656054fb2fb9b31f07631d15f19a"
+DEFAULT_INSTALL_SHA256="133242b850a1e2dfa820f39b466c8b56a9363aee1cc8df1e9a1f9f28744a0cdc"
 MAX_INSTALLER_BYTES=2097152
 
 cleanup() {
diff --git a/tests/test_install.py b/tests/test_install.py
index 7abb9156..fe7f06e6 100644
--- a/tests/test_install.py
+++ b/tests/test_install.py
@@ -2245,7 +2245,7 @@ class CaseVariants(unittest.TestCase):
         compact = " ".join(source.split())
         self.assertIn(
             'variants = manifest.get("case_variants", []) '
-            "if variants and keeps_case(bios_path):", compact,
+            "case_kept = keeps_case(bios_path) if variants and case_kept:", compact,
         )
 
     def test_the_published_retroarch_list_carries_them(self):
@@ -2318,5 +2318,22 @@ class TheBoardTheSystemNamesIsTheTarget(unittest.TestCase):
                 self.assertTrue(targets)
 
 
+
+class CaseTwinsOnAFoldingVolume(unittest.TestCase):
+    """Batocera and Recalbox list np2kai/bios.rom and np2kai/BIOS.ROM, other
+    bytes each. Installed to a share opened from Windows or an exFAT card,
+    the second overwrote the first and --check never converged."""
+
+    def test_one_file_per_name_is_kept(self):
+        files = [{"dest": "np2kai/bios.rom"}, {"dest": "np2kai/BIOS.ROM"}, {"dest": "x.bin"}]
+        kept, folded = install.fold_case_twins(files)
+        self.assertEqual([f["dest"] for f in kept], ["np2kai/bios.rom", "x.bin"])
+        self.assertEqual(folded, ["np2kai/BIOS.ROM"])
+
+    def test_main_folds_only_where_case_is_not_kept(self):
+        source = " ".join((REPO_ROOT / "install.py").read_text(encoding="utf-8").split())
+        self.assertIn("if not case_kept: files, folded = fold_case_twins(files)", source)
+
+
 if __name__ == "__main__":
     unittest.main()
diff --git a/wiki/troubleshooting.md b/wiki/troubleshooting.md
index 4303a16d..1d5b51c5 100644
--- a/wiki/troubleshooting.md
+++ b/wiki/troubleshooting.md
@@ -95,6 +95,13 @@ BIOS directory keeps case and, if it does, also fetches these names. If you
 extract the ZIP on Linux by hand, take the Lakka pack instead: it is the
 RetroArch pack with both spellings.
 
+The reverse happens on Batocera and Recalbox, whose lists hold both spellings
+because their systems keep case: NP2kai reads `np2kai/bios.rom` and
+`np2kai/BIOS.ROM` as two different files. Written to a folder that folds case
+(the share opened from Windows, an exFAT card), one overwrites the other. The
+installer checks the folder first, keeps one file per name there and lists
+the ones it left out.
+
 ## Hash mismatch / UNTESTED
 
 `verify.py` reports `UNTESTED` for a file.