diff --git a/tests/functional/duplicate.nar b/tests/functional/duplicate.nar
new file mode 100644
index 000000000..1d0993ed4
Binary files /dev/null and b/tests/functional/duplicate.nar differ
diff --git a/tests/functional/local.mk b/tests/functional/local.mk
index 8b4945cac..f61823765 100644
--- a/tests/functional/local.mk
+++ b/tests/functional/local.mk
@@ -90,7 +90,7 @@ nix_tests = \
   derivation-advanced-attributes.sh \
   import-derivation.sh \
   nix_path.sh \
-  case-hack.sh \
+  nars.sh \
   placeholders.sh \
   ssh-relay.sh \
   build.sh \
diff --git a/tests/functional/case-hack.sh b/tests/functional/nars.sh
similarity index 79%
rename from tests/functional/case-hack.sh
rename to tests/functional/nars.sh
index feddc6583..c58d12cd5 100755
--- a/tests/functional/case-hack.sh
+++ b/tests/functional/nars.sh
@@ -6,13 +6,14 @@ TODO_NixOS
 
 clearStore
 
-rm -rf "$TEST_ROOT/case"
-
-opts=("--option" "use-case-hack" "true")
+# Check that NARs with duplicate directory entries are rejected.
+rm -rf "$TEST_ROOT/out"
+expectStderr 1 nix-store --restore "$TEST_ROOT/out" < duplicate.nar | grepQuiet "NAR directory is not sorted"
 
 # Check whether restoring and dumping a NAR that contains case
 # collisions is round-tripping, even on a case-insensitive system.
-
+rm -rf "$TEST_ROOT/case"
+opts=("--option" "use-case-hack" "true")
 nix-store "${opts[@]}" --restore "$TEST_ROOT/case" < case.nar
 nix-store "${opts[@]}" --dump "$TEST_ROOT/case" > "$TEST_ROOT/case.nar"
 cmp case.nar "$TEST_ROOT/case.nar"