Unique Feed URL

Hello guys,

there is a problem on offering authenticated users the opportunity to add a feed: Different users can add the same feed URL.

So is it possible to set the entry "Feed URL" the was inserted in the edit form as unique (global)? The purpose of this unique Feed URL is to avoid duplicate feed entries.

I tried it unsuccessfully with the modules https://drupal.org/project/unique_field and https://drupal.org/project/bs_uniquenode.

Alex