diff --git a/pjlib/src/pj/timer.c b/pjlib/src/pj/timer.c
index 8eaab79..2a791b6 100644
--- a/pjlib/src/pj/timer.c
+++ b/pjlib/src/pj/timer.c
@@ -446,6 +446,9 @@
     PJ_ASSERT_RETURN(ht && entry && delay, PJ_EINVAL);
     PJ_ASSERT_RETURN(entry->cb != NULL, PJ_EINVAL);
 
+    /* Prevent same entry from being scheduled more than once */
+    PJ_ASSERT_RETURN(entry->_timer_id < 1, PJ_EINVALIDOP);
+
     pj_gettimeofday(&expires);
     PJ_TIME_VAL_ADD(expires, *delay);