Warn if runner with same token being registered multiple times

What does this MR do?

This MR adds logic to log a warning if the user tries to register a runner with a token that is already present in the config.toml.

Why was this MR needed?

To make it clear to users that an authentication token doesn't behave like a registration token.

What's the best way to test this MR?

Try to register a runner with the same token twice. The following message should appear:

WARNING: A runner with this system ID and token has already been registered.

What are the relevant issue numbers?

Closes #31081 (closed)