Only append to firewall list if needed

src/freedombone-utils-firewall

     iptables -A OUTPUT -o tun+ -j ACCEPT
     save_firewall_settings
 
-    echo "VPN=1194" >> $FIREWALL_CONFIG
+    if ! grep -q "VPN=" $FIREWALL_CONFIG; then
+        echo "VPN=1194" >> $FIREWALL_CONFIG
+    fi
 }
 
 function configure_firewall {