Problem with Mac Mail duplicating subject headers

Hi - this is my first post, so hello to everyone and thanks for reading!

I have an old Intel MacBook running OS X 10.4.11, and my mail browser is Mac Mail. Recently, something very odd has been happening.

In the browser window for my 'In' box and 'Sent' box, the subject field for lots of my emails is all wrong. Instead of their original titles, loads of emails have duplicated subject titles, copied from an unrelated email. For instance, fifteen recent emails suddenly show a subject/title from an earlier email. When I open the messages themselves the subject line remains as it should be; however, when I try to reply to an email with this problem, the wrong subject/title appears in the new subject line.

The wrongly duplicated titles keep changing but I can't see a pattern - this has been happening for a week now. I've tried using 'Rebuild' but if anything it seems slightly worse!

Is this some sort of virus or does the program need reconfiguring somehow?

Any suggestions gratefully received!