Weird Apple mail behavior

My Apple Mail has been acting strange for over a year now. When I check emails in the junk folder, I often select one but when it opens, it has the content of another email [example - I select an email from Amazon but when it opens it has the subject line and content of another email in the junk folder]. If I rebuild the mailbox, it tends to fix the problem, but I have to do this several times a day.

What's going on? Are there any permanent fixes or must I constantly rebuild the mailbox?