Apple Mail on Mac: Incorrect name of sender in mail received

For some weeks now, all the mail that I receive on my MacBook using Mail for one specific sender is incorrectly shown in the list as sent by another person.

The name on the email itself is correct. The error is in the "From" column in the list of emails. (I use the classic layout.)

I think that this problem started when I discovered that I had mistakenly put the same email address on two of my contacts in my Contacts book.

What I have done is:

• I corrected the address book — removed the erroneous email address from the second person
• Have cleared the Previous Recipients for these two people.
• Have run CleanMyMac to clear the caches.
• Have reset the PROM.
• Turned off "Use smart addresses" in preferences — did nothing.
• Have rebuilt that mailbox.
• And, of course, have restarted the MacBook a number of times.

I running the latest update of High Sierra 10.13.6 and have installed all the Safari security updates.

(I have not updated to Catelina because there is an older application — iWeb — that I still need to use.)

How do i correct his annoyance?

Thank you in advance.