Duplicate email detector

[marclaporte]

For various reasons, we can end up with duplicate emails. Duplicates can cause extra work and confusion. Say you don't notice and you reply twice.

How does this happen?

• I am subscribed to a mailing list and my email is also in "to" or "cc", so I get it twice.
• Two people receive an email related to accounting. They follow the process and move said email to the accounting mailbox (A great Cypht feature is to move emails between accounts).

There are cases where you want duplicate emails (and Cypht offers to move or copy an email to a folder):

• One email covers multiple topics, and you move emails to folders (as topics). This could be handled better with tags one day. https://github.com/jasonmunro/cypht/issues/13
• We should figure out a way to exclude intentional duplicates from future reports

In some cases, the duplicate detector may need to be more sophisticated, to find a near duplicate but not exact duplicate.

This could be a background task, so user is not slowed down.

This whole task is low priority, and we won't get to it soon, if ever. But for completeness, I like to have a task here.

[marclaporte]

Until we have something in Cypht, I sometimes run the delete email duplicate tool with https://www.claws-mail.org/