Sending mails via Mutt and Gmail: Duplicates

To get rid of duplicates, unset $record. Gmail's SMTP server stores the message automatically.

Don't save sent messages, Gmail has that function inbuilt, set record = "+[Gmail]/Sent Mail" is unnecessary (and causes the issue you describe).

The Gmail server automatically stores a copy of all sent mail for you, so you do not need mutt to also store a copy. You want to keep the $record setting as that allows you to view the sent folder from mutt via the < shortcut. What you want to do is to set $copy to no to disable mutt's storing of all sent mail:

set record = "+[Gmail]/Sent Mail"
set copy = no

And no, the emails were not actually sent to the recipients twice.