Duplicate error messages

This fix can be better explained in code so I'm commenting the issue in this PR. Here is the link for the comment.
https://github.com/thoughtbot/paperclip/pull/1418/files#r8854478

More discussion can be found here.
https://github.com/thoughtbot/paperclip/pull/1357/files#r6825945

I had sent another PR for the same issue but I was messed up with my old repository so I had to delete the older repository and the older PR #1418. Consider looking at #1418 also.