diff --git a/sys/net/if_vlan.c b/sys/net/if_vlan.c
--- a/sys/net/if_vlan.c
+++ b/sys/net/if_vlan.c
@@ -1715,10 +1715,20 @@
 		ifv->ifv_proto = proto;
 
 		if (ifv->ifv_vid != vid) {
+			int oldvid = ifv->ifv_vid;
+
 			/* Re-hash */
 			vlan_remhash(trunk, ifv);
 			ifv->ifv_vid = vid;
 			error = vlan_inshash(trunk, ifv);
+			if (error) {
+				int ret __diagused;
+
+				ifv->ifv_vid = oldvid;
+				/* Re-insert back where we found it. */
+				ret = vlan_inshash(trunk, ifv);
+				MPASS(ret == 0);
+			}
 		}
 		/* Will unlock */
 		goto done;
diff --git a/tests/sys/net/if_vlan.sh b/tests/sys/net/if_vlan.sh
--- a/tests/sys/net/if_vlan.sh
+++ b/tests/sys/net/if_vlan.sh
@@ -297,6 +297,42 @@
 	vnet_cleanup
 }
 
+atf_test_case "conflict_id" "cleanup"
+conflict_id_head()
+{
+	atf_set descr 'Test conflicting VLAN IDs, PR #279195'
+	atf_set require.user root
+}
+
+conflict_id_body()
+{
+	vnet_init
+
+	epair=$(vnet_mkepair)
+
+	vnet_mkjail alcatraz ${epair}b
+	vlan_a=$(jexec alcatraz ifconfig vlan create)
+	vlan_b=$(jexec alcatraz ifconfig vlan create)
+
+	jexec alcatraz ifconfig ${vlan_a} vlan 100 vlandev ${epair}b
+	jexec alcatraz ifconfig ${vlan_b} vlan 101 vlandev ${epair}b
+
+	atf_check -s exit:1 -o ignore -e ignore \
+	    jexec alcatraz ifconfig ${vlan_a} vlan 101
+
+	atf_check -s exit:0 -o match:"vlan: 100" \
+	    jexec alcatraz ifconfig ${vlan_a}
+
+	atf_check -s exit:0 -o ignore -e ignore \
+	    jexec alcatraz ifconfig ${vlan_a} vlan 100
+}
+
+conflict_id_cleanup()
+{
+	vnet_cleanup
+
+}
+
 atf_init_test_cases()
 {
 	atf_add_test_case "basic"
@@ -306,4 +342,5 @@
 	atf_add_test_case "qinq_dot"
 	atf_add_test_case "qinq_setflags"
 	atf_add_test_case "bpf_pcp"
+	atf_add_test_case "conflict_id"
 }