If you try to start a car that is already running, the following things might occur, depending on the car's type and mechanism:

Grinding Noise from Starter Motor:

The starter motor engages with the engine's flywheel during ignition. If the engine is already running, this engagement can cause a loud grinding or whining noise as the starter gear clashes with the already-moving flywheel.

Damage to the Starter Motor:

Repeatedly attempting to start an already-running engine can damage the starter motor. The high-speed interaction between the starter and flywheel may wear out the starter gears or cause them to break.

Potential Flywheel Damage:

The flywheel, which the starter engages to start the engine, can also sustain damage due to the repeated mechanical impact.

Electrical System Strain:

In older vehicles, engaging the starter motor while the engine is running could strain the electrical system unnecessarily, potentially leading to a blown fuse or battery drain.

Modern Cars with Safety Features:

Many modern vehicles have built-in safety mechanisms that prevent the starter motor from engaging if the engine is already running. Apart from the system rejecting the start attempt, nothing significant will happen in such cars.

Unusual Vibrations or Sounds:

If the car does not have safety mechanisms, you may notice unusual vibrations or sounds, indicating mechanical strain.

Learning Opportunity for Driver:

Such an incident is a reminder to always check the dashboard indicators (e.g., tachometer, or engine light) to confirm the engine's status before attempting to start it.

Always ensure the engine is off before turning the key or pressing the start button to avoid these issues.