I encountered this bug a week ago. Two people on different computers where creating a message at the same time. And submitted exactly at the same time. The messages both got a #9 in the recipient interface. See the attached screenshot.
Thank you for reporting this @Raf_Rasenberg
It was reported once but we were not able to replicate.
Will investigate the issue and get to you.
Already by next release will add a constraint able to protect causing the issue to not happen eventually denying the second duplicated submission to happen as an immediate fix.