Contacts double enters surname

I have a problem with contacts on a single entry name only. Unfortunately it is the name of my wife so I really need it in my contacts. This problem started earlier this year. I tried to fix it then but failed and gave up as other priorities intervened but I recently found some more clues.

In Contacts I enter the name "First Last" but Contacts registers this as "First Last, last" (note no capital on second occurrence). "Last, last" is greyed out and I cannot edit it. This only happens with one EXACT name. Any other name behaves normally. For example if I enter "Firs Last" (or even delete the final "t" in "First") it behaves normally but as soon as I add the "t" the surname switches to greyed out "Last, last" again. When I press Done to finish editing the name displays normally, the greyed out "Last, last" is only visible when editing or entering data.

The problem is very specific. eg, "First Lasx" works normally, in fact any other spelling other than the exact one is OK.

Contacts is clearly finding this information somewhere but I can't figure out where from or how to stop it. Because the name is my wife there are huge numbers of texts, email etc over many years so there are many places it could be coming from. I have tried deleting the contact on both Mac and phone, resync and then enter again but the problem still occurs. Based on other advice I also deleted "Previous recipients" in Mail but no help there either.

Consequences of this problem are that both the Mac and iPhone refuse to forward calendar requests and multiple other problems so I can't ignore it, I need to communicate with my wife!