Multiple listing in List of References in my Word Doc


I am a newbie. Currently I am writing my Masters Thesis and every time I cite the same source it will be listed at the end of my doc. Meaning if I use the same source 5 times, citing it 5 times, it will appear 5 times in my list of references. How can I change it in a way that it only appears once? 

Thank you so much in advance!!!

What output style are you using?  Do you have the record in your library multiple times?