When right-clicking on the seat of a locked Scout, I got double messages each time I clicked. The "This vehicle is locked" message was identical each time.
I've seen this happen a ton when people (including myself, regrettably) forgot to make sure the message only displayed on client side or server side, and not both, so that could be it.

Was this on singleplayer a server, or both?

Singleplayer.

Fixed in V12.3.0.