You are creating a durable function and starting its execution at the same time by calling df.start(<durable function definition>). This will you give you back an instance id which represents this durable function execution. You can use this to refer to this execution from this point onwards.

Within this durable function you are calling df.wait_for_signal(<signal_name>). This call is exactly once within this function instance. There are no duplicates possible. Your df.start() call might get duplicated if it times out and you re-run it, but in this case it would end up creating a different function instance.

Any 'unhandled' errors in executing SQL will fail the function instance. Its status would bubble up the exact error being raised.