Hi, Andrzej!
 
I like that you like it :)  To answer your question: no, in the example I gave (
set route_ref='$(route_ref),${ref|not-contained:,:route_ref}' | '$(route_ref)' | '${ref}';
) there will be no extra comma in case of repeating names. In such a case the first alternative in the rule cannot be applied but the second one ('$(route_ref)') can and so it fires, leaving the value unchanged.
So, if you had a string "1,2" and then get again a ref=2, after applying the filter the string would still be "1,2". If after that you get a ref=3, after applying the filter you get "1,2,3". So you don't have to worry about it.
Actually, I also miss conditional statements in apply block. But for now, as long as it's not possible, this filter could be useful in some situations.
 
Max