diff --git a/src/compton.c b/src/compton.c
index 92d08dc5..7f9b879f 100644
--- a/src/compton.c
+++ b/src/compton.c
@@ -3320,7 +3320,8 @@ xerror(Display __attribute__((unused)) *dpy, XErrorEvent *ev) {
 
   if (ev->request_code == ps->composite_opcode
       && ev->minor_code == X_CompositeRedirectSubwindows) {
-    fprintf(stderr, "Another composite manager is already running\n");
+    fprintf(stderr, "Another composite manager is already running "
+        "(and does not handle _NET_WM_CM_Sn correctly)\n");
     exit(1);
   }
 
@@ -4348,6 +4349,16 @@ ev_screen_change_notify(session_t *ps,
   }
 }
 
+inline static void
+ev_selection_clear(session_t *ps,
+    XSelectionClearEvent __attribute__((unused)) *ev) {
+  // The only selection we own is the _NET_WM_CM_Sn selection.
+  // If we lose that one, we should exit.
+  fprintf(stderr, "Another composite manager started and "
+      "took the _NET_WM_CM_Sn selection.\n");
+  exit(1);
+}
+
 #if defined(DEBUG_EVENTS) || defined(DEBUG_RESTACK)
 /**
  * Get a window's name from window ID.
@@ -4440,6 +4451,9 @@ ev_handle(session_t *ps, XEvent *ev) {
     case PropertyNotify:
       ev_property_notify(ps, (XPropertyEvent *)ev);
       break;
+    case SelectionClear:
+      ev_selection_clear(ps, (XSelectionClearEvent *)ev);
+      break;
     default:
       if (ps->shape_exists && ev->type == ps->shape_event) {
         ev_shape_notify(ps, (XShapeEvent *) ev);
@@ -4892,6 +4906,7 @@ register_cm(session_t *ps) {
   if (!ps->o.no_x_selection) {
     unsigned len = strlen(REGISTER_PROP) + 2;
     int s = ps->scr;
+    Atom atom;
 
     while (s >= 10) {
       ++len;
@@ -4901,7 +4916,13 @@ register_cm(session_t *ps) {
     char *buf = malloc(len);
     snprintf(buf, len, REGISTER_PROP "%d", ps->scr);
     buf[len - 1] = '\0';
-    XSetSelectionOwner(ps->dpy, get_atom(ps, buf), ps->reg_win, 0);
+    atom = get_atom(ps, buf);
+
+    if (XGetSelectionOwner(ps->dpy, atom) != None) {
+      fprintf(stderr, "Another composite manager is already running\n");
+      return false;
+    }
+    XSetSelectionOwner(ps->dpy, atom, ps->reg_win, 0);
     free(buf);
   }