[dba-Tech] Gmail - Eliminating duplicate emails

[Rocky Smolin]

Dear List(s):

I have a family member with a new comp. In his existing email (which is a
COX address) he has an inbox with about 28k emails and an archive box with
about 53k emails.

Many of those emails are duplicates.  How they got that way is not
important. He's moving to Gmail and before he brings these two files of
emails over to his gmail account, he'd like to clean out the duplicates.

I found some simple instructions to do this but the instructions didn't
delete any emails.

There are products you can buy for this specific purpose but all of them
seem to have their detractors.

Any guidance? Has anyone had to do this kind of cleanup before?

MTIA

ROcky