diff --git a/src/libutil/archive.cc b/src/libutil/archive.cc
index d20936de4b4..39af6dc8360 100644
--- a/src/libutil/archive.cc
+++ b/src/libutil/archive.cc
@@ -23,7 +23,7 @@ struct ArchiveSettings : Config
             false,
         #endif
         "use-case-hack",
-        "Whether to enable a Darwin-specific hack for dealing with file name collisions."};
+        "Whether to enable a macOS-specific hack for dealing with file name case collisions."};
 };
 
 static ArchiveSettings archiveSettings;
@@ -214,11 +214,13 @@ static void parse(FileSystemObjectSink & sink, Source & source, const Path & pat
             else if (t == "directory") {
                 sink.createDirectory(path);
 
+                std::string prevName;
+
                 while (1) {
                     s = getString();
 
                     if (s == "entry") {
-                        std::string name, prevName;
+                        std::string name;
 
                         s = getString();
                         if (s != "(") throw badArchive("expected open tag");
@@ -241,6 +243,9 @@ static void parse(FileSystemObjectSink & sink, Source & source, const Path & pat
                                         debug("case collision between '%1%' and '%2%'", i->first, name);
                                         name += caseHackSuffix;
                                         name += std::to_string(++i->second);
+                                        auto j = names.find(name);
+                                        if (j != names.end())
+                                            throw Error("NAR contains file name '%s' that collides with case-hacked file name '%s'", prevName, j->first);
                                     } else
                                         names[name] = 0;
                                 }
diff --git a/tests/functional/case-collision.nar b/tests/functional/case-collision.nar
new file mode 100644
index 00000000000..2eff86901c6
Binary files /dev/null and b/tests/functional/case-collision.nar differ
diff --git a/tests/functional/case-hack.sh b/tests/functional/case-hack.sh
deleted file mode 100755
index fbc8242ff8f..00000000000
--- a/tests/functional/case-hack.sh
+++ /dev/null
@@ -1,21 +0,0 @@
-#!/usr/bin/env bash
-
-source common.sh
-
-clearStore
-
-rm -rf $TEST_ROOT/case
-
-opts="--option use-case-hack true"
-
-# Check whether restoring and dumping a NAR that contains case
-# collisions is round-tripping, even on a case-insensitive system.
-nix-store $opts  --restore $TEST_ROOT/case < case.nar
-nix-store $opts --dump $TEST_ROOT/case > $TEST_ROOT/case.nar
-cmp case.nar $TEST_ROOT/case.nar
-[ "$(nix-hash $opts --type sha256 $TEST_ROOT/case)" = "$(nix-hash --flat --type sha256 case.nar)" ]
-
-# Check whether we detect true collisions (e.g. those remaining after
-# removal of the suffix).
-touch "$TEST_ROOT/case/xt_CONNMARK.h~nix~case~hack~3"
-(! nix-store $opts --dump $TEST_ROOT/case > /dev/null)
diff --git a/tests/functional/duplicate.nar b/tests/functional/duplicate.nar
new file mode 100644
index 00000000000..1d0993ed4ca
Binary files /dev/null and b/tests/functional/duplicate.nar differ
diff --git a/tests/functional/local.mk b/tests/functional/local.mk
index b379eeefee4..bdb47c2db76 100644
--- a/tests/functional/local.mk
+++ b/tests/functional/local.mk
@@ -108,7 +108,7 @@ nix_tests = \
   derivation-json.sh \
   import-derivation.sh \
   nix_path.sh \
-  case-hack.sh \
+  nars.sh \
   placeholders.sh \
   ssh-relay.sh \
   build.sh \
diff --git a/tests/functional/nars.sh b/tests/functional/nars.sh
new file mode 100755
index 00000000000..eb375701635
--- /dev/null
+++ b/tests/functional/nars.sh
@@ -0,0 +1,92 @@
+#!/usr/bin/env bash
+
+source common.sh
+
+clearStore
+
+# Check that NARs with duplicate directory entries are rejected.
+rm -rf "$TEST_ROOT/out"
+expectStderr 1 nix-store --restore "$TEST_ROOT/out" < duplicate.nar | grepQuiet "NAR directory is not sorted"
+
+# Check that nix-store --restore fails if the output already exists.
+expectStderr 1 nix-store --restore "$TEST_ROOT/out" < duplicate.nar | grepQuiet "creating directory '.*/out': File exists"
+
+rm -rf "$TEST_ROOT/out"
+echo foo > "$TEST_ROOT/out"
+expectStderr 1 nix-store --restore "$TEST_ROOT/out" < duplicate.nar | grepQuiet "creating directory '.*/out': File exists"
+
+rm -rf "$TEST_ROOT/out"
+ln -s "$TEST_ROOT/out2" "$TEST_ROOT/out"
+expectStderr 1 nix-store --restore "$TEST_ROOT/out" < duplicate.nar | grepQuiet "creating directory '.*/out': File exists"
+
+mkdir -p "$TEST_ROOT/out2"
+expectStderr 1 nix-store --restore "$TEST_ROOT/out" < duplicate.nar | grepQuiet "creating directory '.*/out': File exists"
+
+# The same, but for a regular file.
+nix-store --dump ./nars.sh > "$TEST_ROOT/tmp.nar"
+
+rm -rf "$TEST_ROOT/out"
+nix-store --restore "$TEST_ROOT/out" < "$TEST_ROOT/tmp.nar"
+expectStderr 1 nix-store --restore "$TEST_ROOT/out" < "$TEST_ROOT/tmp.nar" | grepQuiet "File exists"
+
+rm -rf "$TEST_ROOT/out"
+mkdir -p "$TEST_ROOT/out"
+expectStderr 1 nix-store --restore "$TEST_ROOT/out" < "$TEST_ROOT/tmp.nar" | grepQuiet "File exists"
+
+rm -rf "$TEST_ROOT/out"
+ln -s "$TEST_ROOT/out2" "$TEST_ROOT/out"
+expectStderr 1 nix-store --restore "$TEST_ROOT/out" < "$TEST_ROOT/tmp.nar" | grepQuiet "File exists"
+
+mkdir -p "$TEST_ROOT/out2"
+expectStderr 1 nix-store --restore "$TEST_ROOT/out" < "$TEST_ROOT/tmp.nar" | grepQuiet "File exists"
+
+# The same, but for a symlink.
+ln -sfn foo "$TEST_ROOT/symlink"
+nix-store --dump "$TEST_ROOT/symlink" > "$TEST_ROOT/tmp.nar"
+
+rm -rf "$TEST_ROOT/out"
+nix-store --restore "$TEST_ROOT/out" < "$TEST_ROOT/tmp.nar"
+[[ -L "$TEST_ROOT/out" ]]
+expectStderr 1 nix-store --restore "$TEST_ROOT/out" < "$TEST_ROOT/tmp.nar" | grepQuiet "File exists"
+
+rm -rf "$TEST_ROOT/out"
+mkdir -p "$TEST_ROOT/out"
+expectStderr 1 nix-store --restore "$TEST_ROOT/out" < "$TEST_ROOT/tmp.nar" | grepQuiet "File exists"
+
+rm -rf "$TEST_ROOT/out"
+ln -s "$TEST_ROOT/out2" "$TEST_ROOT/out"
+expectStderr 1 nix-store --restore "$TEST_ROOT/out" < "$TEST_ROOT/tmp.nar" | grepQuiet "File exists"
+
+mkdir -p "$TEST_ROOT/out2"
+expectStderr 1 nix-store --restore "$TEST_ROOT/out" < "$TEST_ROOT/tmp.nar" | grepQuiet "File exists"
+
+# 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 "$TEST_ROOT/case.nar"
+[ "$(nix-hash "${opts[@]}" --type sha256 "$TEST_ROOT/case")" = "$(nix-hash --flat --type sha256 case.nar)" ]
+
+# Check whether we detect true collisions (e.g. those remaining after
+# removal of the suffix).
+touch "$TEST_ROOT/case/xt_CONNMARK.h~nix~case~hack~3"
+(! nix-store "${opts[@]}" --dump "$TEST_ROOT/case" > /dev/null)
+
+# Detect NARs that have a directory entry that after case-hacking
+# collides with another entry (e.g. a directory containing 'Test',
+# 'Test~nix~case~hack~1' and 'test').
+rm -rf "$TEST_ROOT/case"
+expectStderr 1 nix-store "${opts[@]}" --restore "$TEST_ROOT/case" < case-collision.nar | grepQuiet "NAR contains file name 'test' that collides with case-hacked file name 'Test~nix~case~hack~1'"
+
+# Deserializing a NAR that contains file names that Unicode-normalize
+# to the same name should fail on macOS but succeed on Linux.
+rm -rf "$TEST_ROOT/out"
+if [[ $(uname) = Darwin ]]; then
+    expectStderr 1 nix-store --restore "$TEST_ROOT/out" < unnormalized.nar | grepQuiet "File exists"
+else
+    nix-store --restore "$TEST_ROOT/out" < unnormalized.nar
+    [[ -e $TEST_ROOT/out/â ]]
+    [[ -e $TEST_ROOT/out/â ]]
+fi
diff --git a/tests/functional/unnormalized.nar b/tests/functional/unnormalized.nar
new file mode 100644
index 00000000000..4b7edb17e0b
Binary files /dev/null and b/tests/functional/unnormalized.nar differ