why endnote creates multiple lists of references?

Hello everyone, 

My problem is that endnote keeps on creating multiple lists of references, almost with the same content. So I end up with almost the same list of references typed again and again and again at the end of my document. The worst thing is that I can not  be sure which one of them is the most recently updated and so, I have to go through all of them in order to decide which are the extra ones and delete them. It is like, every time that I hit the “update Bibliography” button, endnote creates a copy of all the references from the begining and also keeps all the previous ones.

Any suggestions?

Thanks!!!