Duplicate images in Galleries that are NOT in the media Library

HI there, Im new to wordpress and am really enjoying it.
I am a photograpehr who is building his first wordpores site having become tired of the boring bulk template site builders uot there.

I am using the GrandPhotographyVersion: 2.1 theme.
I have noticed that in my Galleries I am getting many images displayed amultiple times. I have done some looking on forums and there does seem to be an issue of duplicate images accross wordpress.
I have looked in my media library on my dashboard and none apprear to have duplicated. and yet some images appear up to 3 times in a gallery on a webpage.
IS there a reason for this? ANd how can it be prevented?

Thanks for your help