diff --git a/docker/routlin-dash/app/factory.py b/docker/routlin-dash/app/factory.py
index 2e0021f..068be29 100644
--- a/docker/routlin-dash/app/factory.py
+++ b/docker/routlin-dash/app/factory.py
@@ -209,6 +209,7 @@ for(var i=0;i<lines.length;i++){
   var r=_checkLine(l);
   if(!r.ok&&!r.partial)return r;
   if(!r.ok){hasPartial=true;continue;}
+  if(collisions&&collisions.some(function(c){return String(c)===l;}))return _err('Already in use');
   if(dedup){if(seen[l])return _err('Duplicate entry');seen[l]=true;}
 }
 if(!hasContent)return _par('');