De-duplicate addresses in recipient fields

Please search to check for an existing issue (including closed issues, for which the fix may not have yet been released) before opening a new issue: https://github.com/k9mail/k-9/issues?q=is%3Aissue

Expected behavior

Tell us what should happen

I reply all to an email in which the sender has cc'd him or herself. This results in the sender's email showing up twice in the header. I expect the delicate to be ignored, as it is in every other email client I've used in the past 20 years or more.

Actual behavior

Tell us what happens instead

Message gets stuck in outbox. At least the error message explains it and I can edit it again to remove the duplicate and resend.

Steps to reproduce

1. Reply all to an email on which the sender has cc'd herself
2. Note that sender's email is in the header twice, once in cc, once in to
3. Send, now that message is stuck in outbox with an error

Environment

K-9 Mail version:
5.714
Android version:
10
Account type (IMAP, POP3, WebDAV/Exchange):
Imap
Please take some time to retrieve logs and attach them here:

Start by reproducing the reply-all scenario described for K-9 Mail 5.714 on Android 10 with an IMAP account. Trace recipient handling and the send failure from that reproduction; done means duplicate addresses are removed before sending and the message no longer remains stuck in the outbox.