diff --git a/src/core/netif.c b/src/core/netif.c
index 4e888f44..1d668e27 100644
--- a/src/core/netif.c
+++ b/src/core/netif.c
@@ -379,6 +379,7 @@ netif_add(struct netif *netif,
       }
       num_netifs = 0;
       for (netif2 = netif_list; netif2 != NULL; netif2 = netif2->next) {
+        LWIP_ASSERT("netif already added", netif2 != netif);
         num_netifs++;
         LWIP_ASSERT("too many netifs, max. supported number is 255", num_netifs <= 255);
         if (netif2->num == netif->num) {