Two same files in gallery but just one showing

Hi.
I am experiencing this issue for a quite a long time. Each time I upload some photos to gallery it will show all of them + some of them got another copy with same name but no thumbnail. You can see screenshot on this link: http://imageshack.com/a/img905/5246/YHm4fX.jpg
I am pretty sure it has nothing to do with theme couse it does the same on more of my sites.
Anybody has any idea why is that?

https://wordpress.org/plugins/gallery/