diff --git a/src/system/session_manager.vala b/src/system/session_manager.vala
index ebb74b7..164c12a 100644
--- a/src/system/session_manager.vala
+++ b/src/system/session_manager.vala
@@ -25,12 +25,28 @@ namespace Singularity {
          */
         public signal void session_ending();
 
+        private Subprocess? lockscreen_process = null;
+
+        public bool locked { get; private set; default = false; }
+
         public void lock_screen() {
+            if (lockscreen_process != null) return;
+
             // 1. Native singularity-lockscreen (ext-session-lock-v1 + PAM)
             string? lockscreen = find_singularity_binary("singularity-lockscreen");
             if (lockscreen != null) {
                 try {
-                    Process.spawn_command_line_async(lockscreen);
+                    var process = new Subprocess(SubprocessFlags.NONE, lockscreen);
+                    lockscreen_process = process;
+                    locked = true;
+                    process.wait_async.begin(null, (obj, res) => {
+                        try {
+                            process.wait_async.end(res);
+                        } catch (Error e) { }
+                        if (lockscreen_process != process) return;
+                        lockscreen_process = null;
+                        locked = false;
+                    });
                     return;
                 } catch (Error e) {
                     warning("singularity-lockscreen failed: %s", e.message);
diff --git a/tests/power_test.vala b/tests/power_test.vala
index 52dc591..d561056 100644
--- a/tests/power_test.vala
+++ b/tests/power_test.vala
@@ -227,6 +227,33 @@ private void test_charge_limit() {
     assert(replaced.charge_limit_available);
 }
 
+private void test_lock_screen_single_instance() {
+    string count = Path.build_filename(fixture_root, "lock-starts");
+    string release = Path.build_filename(fixture_root, "lock-release");
+    script("singularity-lockscreen",
+        "echo x >> '%s'\nwhile [ ! -e '%s' ]; do sleep 0.05; done\n".printf(count, release));
+    Environment.set_variable("PATH",
+        Path.build_filename(fixture_root, "bin") + ":" + (Environment.get_variable("PATH") ?? "/usr/bin:/bin"), true);
+
+    var session = Singularity.SessionManager.get_default();
+    session.lock_screen();
+    session.lock_screen();
+    assert(session.locked);
+    wait_for(() => read_file(count) != "");
+    assert(read_file(count) == "x");
+
+    write_file(release, "");
+    wait_for(() => !session.locked);
+    assert(!session.locked);
+    FileUtils.remove(release);
+
+    session.lock_screen();
+    wait_for(() => read_file(count) == "x\nx");
+    assert(read_file(count) == "x\nx");
+    write_file(release, "");
+    wait_for(() => !session.locked);
+}
+
 public static int main(string[] args) {
     Test.init(ref args);
     try {
@@ -238,5 +265,6 @@ public static int main(string[] args) {
     Test.add_func("/power/command-suspend", test_command_suspend);
     Test.add_func("/power/battery-scan", test_battery_scan);
     Test.add_func("/power/charge-limit", test_charge_limit);
+    Test.add_func("/power/lock-screen-single-instance", test_lock_screen_single_instance);
     return Test.run();
 }