Limit entity reference widget to unique references I have a entity-reference field that enables multiple-references (cardinality is set to -1). The problem is that it allows multiple 'same' references => it is possible to save two references to one specific node.

I know I can limit that using my own validation function but I have this feeling that there has to be easier way (e.g. some settings that I don't know about).

If there's none then sorry for useless question.
I don't think the Entity Reference module ships with this feature. But the entityreference unique module should be sufficient for your need. It is in alpha mode, so I would check the issue queue to see if there are any security/critical issues with it before using it on a production env.