Starter trying to restart while idoling

Something is completing the circuit that the starter solenoid connects when you are in "start" position, but you are not turning the key. Therefore juice is getting to the small wire on the soleniod, causing it to kick the starter gear and engage the starter.

Check ignition switch (disengage your airbag deploy circuits first if you remove it). You should only have juice going to the start circuit when the key is turned against the spring. Anything else is a failure of your switch.

The other possibility is that a short is causing juice to go to your solenoid switch - small terminal from relay- if theres a relay it could be on the blink- loose contacts can short, or you have a crossed wire short that is putting juice into your relay or solenoid circuit causing it to engage. Or the solenoid itself may have a loose piece inside that makes contact internally from the pos batt connection to the small wire on the solenoid.

You need to get it fixed as every time it engages while it is running you are grinding gears on your flywheel and starter. The ignition switch, solenoid, and relays will all be cheaper and easier fix than replacing those.

Sounds like either an ignition problem. (Switch, not the lock cylinder) or a short somewhere between the switch and the starter relay.