Shared Folder items showing in user sent items

We have an issue here where Shared Folders are in operation (and there are a few of them). Everything has been working ok until about a month ago and I have 2 post offices being affected at the moment.

Users file items in the shared folder for the whole Team to see. The problem is that these items are appearing in user Sent Items. So one user in their Sent items may have emails from other people (that have come from the shared folders).

thanks in advance