diff --git a/src/commands.c b/src/commands.c
index dc60e974..79071d6b 100644
--- a/src/commands.c
+++ b/src/commands.c
@@ -1043,32 +1043,34 @@ void cmd_workspace_name(I3_CMD, char *name) {
 void cmd_mark(I3_CMD, char *mark, char *toggle) {
     HANDLE_EMPTY_MATCH;
 
-    owindow *current;
-    TAILQ_FOREACH(current, &owindows, owindows) {
-        DLOG("matching: %p / %s\n", current->con, current->con->name);
-        current->con->mark_changed = true;
-        if (toggle != NULL && current->con->mark && strcmp(current->con->mark, mark) == 0) {
-            DLOG("removing window mark %s\n", mark);
-            FREE(current->con->mark);
-        } else {
-            DLOG("marking window with str %s\n", mark);
-            FREE(current->con->mark);
-            current->con->mark = sstrdup(mark);
-        }
+    owindow *current = TAILQ_FIRST(&owindows);
+    if (current == NULL) {
+        ysuccess(false);
+        return;
+    }
+
+    /* Marks must be unique, i.e., no two windows must have the same mark. */
+    if (current != TAILQ_LAST(&owindows, owindows_head)) {
+        yerror("A mark must not be put onto more than one window");
+        return;
+    }
+
+    DLOG("matching: %p / %s\n", current->con, current->con->name);
+    current->con->mark_changed = true;
+    if (toggle != NULL && current->con->mark && strcmp(current->con->mark, mark) == 0) {
+        DLOG("removing window mark %s\n", mark);
+        FREE(current->con->mark);
+    } else {
+        DLOG("marking window with str %s\n", mark);
+        FREE(current->con->mark);
+        current->con->mark = sstrdup(mark);
     }
 
     DLOG("Clearing all non-matched windows with this mark\n");
     Con *con;
     TAILQ_FOREACH(con, &all_cons, all_cons) {
-        /* Skip matched windows, we took care of them already. */
-        bool matched = false;
-        TAILQ_FOREACH(current, &owindows, owindows) {
-            if (current->con == con) {
-                matched = true;
-                break;
-            }
-        }
-        if (matched)
+        /* Skip matched window, we took care of it already. */
+        if (current->con == con)
             continue;
 
         if (con->mark && strcmp(con->mark, mark) == 0) {
diff --git a/testcases/lib/i3test.pm b/testcases/lib/i3test.pm
index c149cbd1..ac1a26ca 100644
--- a/testcases/lib/i3test.pm
+++ b/testcases/lib/i3test.pm
@@ -606,7 +606,7 @@ sub get_dock_clients {
 
 =head2 cmd($command)
 
-Sends the specified command to i3.
+Sends the specified command to i3 and returns the output.
 
   my $ws = unused_workspace;
   cmd "workspace $ws";
diff --git a/testcases/t/210-mark-unmark.t b/testcases/t/210-mark-unmark.t
index 0083547f..99fc92c8 100644
--- a/testcases/t/210-mark-unmark.t
+++ b/testcases/t/210-mark-unmark.t
@@ -141,4 +141,21 @@ cmd 'mark --toggle important';
 is(get_mark_for_window_on_workspace($tmp, $first), 'important', 'left container has the mark now');
 ok(!get_mark_for_window_on_workspace($tmp, $second), 'second containr no longer has the mark');
 
+##############################################################
+# 9: try to mark two cons with the same mark and check that
+#    it fails
+##############################################################
+
+my $first = open_window(wm_class => 'iamnotunique');
+my $second = open_window(wm_class => 'iamnotunique');
+
+my $result = cmd "[instance=iamnotunique] mark important";
+
+is($result->[0]->{success}, 0, 'command was unsuccessful');
+is($result->[0]->{error}, 'A mark must not be put onto more than one window', 'correct error is returned');
+ok(!get_mark_for_window_on_workspace($tmp, $first), 'first container is not marked');
+ok(!get_mark_for_window_on_workspace($tmp, $second), 'second containr is not marked');
+
+##############################################################
+
 done_testing;