Seems like this is actually the default behaviour for a systemd timer, ha.

"Note that in case the unit to activate is already active at the time the timer elapses it is not restarted, but simply left running."

On the flip side I wonder if you can tell it to timeout and kill the service after a while, as I can equally see them getting stuck and never running again, a problem you don't have with the alternative method :-)

Ah, choices...