Rsync question

After advice I received on this excellent forum I plan to start using rsync rather than SFTP to back up remotely.

My question is if I start the job from a crontab, and the following day the session is still backing up what happens when the crontab runs again?

Will I end up with two different rsync proccesess at different points of the backup running?

I just want to make sure I don't get corrupt data or files missed on a backup.