Starting an engine which is already on and running

When the engine is running, the rotating flywheel teeth will impact against the starter gear teeth. This will usually spin the starter teeth in a way the starter bendix will retract that gear. It probably won't damage things but there is no reason it should happening "multiple times". Those engine are remarkably quiet so the driver might look at the tachometer to see it the engine is running. Also, if the key is in the "run" position, the engine is probably already running.

The noise alone should make the operator not want to do that again, If the starter actually fully engaged the ring gear the gear ratio would be about 300 to 1 and the unbalanced starter motor actually blows up.

I agree with Hornet. My hearing is no good, in the past I have hit the crank position when engine was already running, It made that terrible noise where the starter drive gear is hitting the flywheel, very embarrassing, for me. It didn't hurt anything. My vehicle, at the time, was a different year--make--model. I haven't done that in a long time.