Mozilla Thunderbird has recently started duplicating my incoming messages

This happened for a while and then stopped but after a month or so with all OK I am again getting all incoming messages in duplicate with same timestamp.

I do have several machines with T/Bird clients set up and working, ie office, home, laptop etc. and all are on my private subnet. It is possible however that another machine on a different subnet has my email client account installed and inadvertently not logged out but this should not cause this problem. I cannot think of anything else.

Not a disaster but any ideas where I should look for cause?

There are two reasons I discovered so far for similar behavior (using IMAP):

  1. Corrupted index files (.msf)
    There is a repair option in the folder properties of a mail folder.

  2. Changes in the original email
    e.g. I sometimes remove files annexed to an email on my PC-Client. This causes the email to get a new message number and I will receive the same email (but without the annex) on my smartphone email client again. Yet I have not found how to avoid this.

Many thanks,
You have given me a possible clue. I shall check on all my machines as the corrupted index files seem to be the likely cause.