Car engines these days are so silent that i’m often confused if its running or not. Many a times I have turned the ignition on even when the engine is running and its makes a painful metallic sweaky noise. Question is - is it harmful for the engine ?
It is not good for the starter or ring gear attached to the back of the engine.
In the 1990’s manufactures developed a feature sometimes called double-start over-ride, the engine computer will not allow the starter to operate if the engine is running. I am surprised that there are cars today that do not have that feature.
If the key is in the ignition and the engine may be running, before attempting to start the engine look at the tachometer to see if the engine has stopped.