When this happens, in the 'sent' folder in your Thunderbird do you see just one copy? In the 'sent' folder on the server do you see just one? I'm wondering where the duplication is happening, i.e. whether your Thunderbird is sending eight times or whether it's happening on the server. If it appears to be on your Thunderbird, try running it in Thunderbird safe mode and see if the behaviour persists. Also maybe ask your wife to forward you a message with N attachments and see if you receive it N+1 times.

Good questions which I should have addressed. In the sent folder, there is just one sent. Also, she has an Android smart-phone and it only shows the one message with the 7 attachments correctly, so it appears that the duplication is somehow being done in her laptop. I will try the Safe mode and see what happens. I will also try using her T-bird to send me an email with 2 attachments and see what happens. Thanks for the thoughts.

Many thanks to AManchesterman. I found the problem did not occur in Safe Mode. I then disabled all the add-ons and ran in normal mode and the problem was gone. In the add-on manager, I enabled the Adobe add-on and the problem was back. Simply going to leave it disabled. Don't know why it suddenly started to cause this, but does not seem to matter, as I checked that attached PDF files will open and they do.