Index: sys/kern/kern_event.c
===================================================================
--- sys/kern/kern_event.c
+++ sys/kern/kern_event.c
@@ -680,11 +680,14 @@
 	struct proc *p;
 	struct knote *kn;
 	int cpuid;
+	int flags;
 	TAILQ_ENTRY(kq_timer_cb_data) link;
 	sbintime_t next;	/* next timer event fires at */
 	sbintime_t to;		/* precalculated timer period, 0 for abs */
 };
 
+#define	KQ_TIMER_CB_ENQUEUED	0x01
+
 static void
 kqtimer_sched_callout(struct kq_timer_cb_data *kc)
 {
@@ -706,6 +709,7 @@
 
 	TAILQ_FOREACH_SAFE(kc, &p->p_kqtim_stop, link, kc1) {
 		TAILQ_REMOVE(&p->p_kqtim_stop, kc, link);
+		kc->flags &= ~KQ_TIMER_CB_ENQUEUED;
 		if (kc->next <= now)
 			filt_timerexpire_l(kc->kn, true);
 		else
@@ -753,7 +757,10 @@
 		if (!proc_locked)
 			PROC_LOCK(p);
 		if (P_SHOULDSTOP(p) || P_KILLED(p)) {
-			TAILQ_INSERT_TAIL(&p->p_kqtim_stop, kc, link);
+			if ((kc->flags & KQ_TIMER_CB_ENQUEUED) == 0) {
+				kc->flags |= KQ_TIMER_CB_ENQUEUED;
+				TAILQ_INSERT_TAIL(&p->p_kqtim_stop, kc, link);
+			}
 			if (!proc_locked)
 				PROC_UNLOCK(p);
 			return;
@@ -826,6 +833,7 @@
 	kc->kn = kn;
 	kc->p = curproc;
 	kc->cpuid = PCPU_GET(cpuid);
+	kc->flags = 0;
 	callout_init(&kc->c, 1);
 	filt_timerstart(kn, to);
 
@@ -856,6 +864,11 @@
 
 	kc = kn->kn_ptr.p_v;
 	callout_drain(&kc->c);
+	if ((kc->flags & KQ_TIMER_CB_ENQUEUED) != 0) {
+		PROC_LOCK(kc->p);
+		TAILQ_REMOVE(&kc->p->p_kqtim_stop, kc, link);
+		PROC_UNLOCK(kc->p);
+	}
 	free(kc, M_KQUEUE);
 	old = atomic_fetchadd_int(&kq_ncallouts, -1);
 	KASSERT(old > 0, ("Number of callouts cannot become negative"));