diff --git a/convert/common.c b/convert/common.c
index dbf0c96d..5e7a1d6b 100644
--- a/convert/common.c
+++ b/convert/common.c
@@ -104,11 +104,10 @@ static int setup_temp_super(int fd, struct btrfs_mkfs_config *cfg,
 			ret = -EINVAL;
 			goto out;
 		}
-		if (!test_uuid_unique(cfg->fs_uuid)) {
-			error("non-unique UUID: %s", cfg->fs_uuid);
-			ret = -EINVAL;
-			goto out;
-		}
+		/*
+		 * Caller should make sure the uuid is either unique or OK to
+		 * be duplicate in case it's copied from the source filesystem.
+		 */
 		uuid_copy(super->metadata_uuid, super->fsid);
 	} else {
 		uuid_generate(super->fsid);
diff --git a/convert/main.c b/convert/main.c
index 68f85988..c162e8d8 100644
--- a/convert/main.c
+++ b/convert/main.c
@@ -1189,6 +1189,8 @@ static int do_convert(const char *devname, u32 convert_flags, u32 nodesize,
 
 	if (convert_flags & CONVERT_FLAG_COPY_FSID) {
 		uuid_unparse(cctx.fs_uuid, mkfs_cfg.fs_uuid);
+		if (!test_uuid_unique(mkfs_cfg.fs_uuid))
+			warning("non-unique UUID (copy): %s", mkfs_cfg.fs_uuid);
 	} else if (fsid[0] == 0) {
 		uuid_t uuid;
 
@@ -1196,6 +1198,8 @@ static int do_convert(const char *devname, u32 convert_flags, u32 nodesize,
 		uuid_unparse(uuid, mkfs_cfg.fs_uuid);
 	} else {
 		memcpy(mkfs_cfg.fs_uuid, fsid, BTRFS_UUID_UNPARSED_SIZE);
+		if (!test_uuid_unique(mkfs_cfg.fs_uuid))
+			warning("non-unique UUID (user set): %s", mkfs_cfg.fs_uuid);
 	}
 
 	printf("Source filesystem:\n");
diff --git a/tests/convert-tests/021-uuid-fsid/test.sh b/tests/convert-tests/021-uuid-fsid/test.sh
index d663513f..692a667c 100755
--- a/tests/convert-tests/021-uuid-fsid/test.sh
+++ b/tests/convert-tests/021-uuid-fsid/test.sh
@@ -5,9 +5,12 @@ source "$TEST_TOP/common"
 source "$TEST_TOP/common.convert"
 
 setup_root_helper
-prepare_test_dev
 check_prereq btrfs-convert
 check_global_prereq mke2fs
+setup_loopdevs 1
+prepare_loopdevs
+# Convert helpers need the backing file, can't pass ${loopdevs[1]}
+TEST_DEV=${loopdev_prefix}1
 
 convert_test_prep_fs ext4 mke2fs -t ext4 -b 4096
 run_check_umount_test_dev
@@ -55,3 +58,5 @@ run_check "$TOP/btrfs-convert" --rollback "$TEST_DEV"
 
 run_mustfail "invalid UUID passed" \
 	"$TOP/btrfs-convert" --uuid invalid "$TEST_DEV"
+
+cleanup_loopdevs