2 lines between returns

When i am using Microsoft Word sometimes when i hit the return key the document starts jumping two lines instead of one. This is really irraatating as it makes the document look unprofessional. I am not hitting the return key twice (honest).
I knowthat this is something simple, but i have tried looking in all the paragraph default options etc.
Another thing to not is that when i press return it works fine it is mainly in documents that i have copied and pasted into the documents i am workingon.

Cheers for any help advice