Vacation RR - Duplicate

I love absolutely "I'm going on vacation RR" in postcrossing forum.  I'm one of the 20 top users and was able to host 11 groups.  Amazing concept to receive cards from different places where people go on vacation.  And by hosting a group you offer to send cards from your vacation, and in return you can ask anything, or almost anything. Some users very strict on what they want in return, I always thought that by just asking country/city single view in return is fair enough and should be easy to follow. However my last vacation where I sent 10 postcards from Las Vegas brought me 2 duplicate cards... from people who were in my other vacation group about a year ago.   Needless to say I'm little disappointed and now that got me thinking... may be I need to be more specific... may be I need to make people check my received cards? 

Any thoughts?

received:  February 2013

Exactly same postcard received last July  see here 

Another one:  received February 2013 

\

And another one: October 2012