use replace

files/bridge-post-scripts/post-up.sh.j2

@@ -35,7 +35,7 @@ handle_gateway() {
     GATEWAY_ADDRESS=$(subnetcalc "$BRIDGE_IP" -n | grep "Host Range" | awk '{print $7}')
   fi
 
-  ip route add default via "$GATEWAY_ADDRESS" table "$GATEWAY_NEXT_HOP_MARK"
+  ip route replace default via "$GATEWAY_ADDRESS" table "$GATEWAY_NEXT_HOP_MARK"
 {% if br.type != "dhcp" %}
   ip route add default via "$GATEWAY_ADDRESS" metric "$GATEWAY_NEXT_HOP_MARK"
 {% endif %}

files/ppp-post-scripts/postup.sh.j2

@@ -16,7 +16,7 @@ ip6tables -t mangle -A FORWARD -i "$PPP_IFACE" -p tcp -m tcp --tcp-flags SYN,RST
 
 iptables -t nat -o "$PPP_IFACE" -A POSTROUTING -j MASQUERADE
 
-ip route add default dev "$PPP_IFACE" table $PPPOE_NEXT_HOP_MARK
+ip route replace default dev "$PPP_IFACE" table $PPPOE_NEXT_HOP_MARK
 ip route add default dev "$PPP_IFACE" metric $PPPOE_NEXT_HOP_MARK
 ip rule add pref 100 fwmark $PPPOE_NEXT_HOP_MARK lookup $PPPOE_NEXT_HOP_MARK
 ip rule add pref 400 fwmark $PPPOE_SELECTION_MARK lookup $PPPOE_NEXT_HOP_MARK