Suppression calculated incorrectly

[dragoangel]

If you send 2 emails at once to one user, both of them has hard fails they will be counted separately, this maybe fine, but when one failed email in queue added email to suppression, second will still try to be delivered without checking that this mail already in suppression list and also after N fails will add same email second time to same suppression list.

[catphish]

If duplicate entries are occurring then it is because this code is being executed twice at near enough the same time.
https://github.com/postalserver/postal/blob/main/lib/postal/message_db/suppression_list.rb#L9..L17
If this isn't enough to prevent duplicates then the SELECT+INSERT in this method to be changed to an INSERT … ON DUPLICATE KEY UPDATE command. This will require a unique index to be added.