It seems to depend on the Ping the player has to the server. For me it kept repeating until my ping went down to like 40-50ms where it still scanned once too often.
304ms -> repeats 7-8times more than wanted
243ms -> repeats 2-4times
120ms -> repeats 2 times
<80ms -> repeats 1 time
That's the results I've got after testing a short time.

As I'm not sure where to file bugs, I'll post it here.

I recently noticed that the DNA Scanner kept repeating scans even after I turned Auto-Repeat off. I hope that this can be fixed as it hindered my process of finding a traitor as detective.