Repeating posts in RSS feeds

I am having this problem on a few WordPress sites but must not be searching correctly for the issue here.

In a lot of feedreaders my posts are showing up multiple times. As in once someone has read the post and marked it as read… it will show up again as new next time the feed is checked and I have gotten several comments from people saying this is happening.

I have checked my feed with the Feed Validator and it validates correctly.

Any suggestions?