Drive Moment - Do You Have A "Back-Up Partner", In Case Your Relationship Fails?

Studies have shown that many people, while they are in a steady relationship with someone, also have a person on standby, as a potential romantic option, in case their current relationship fails. It's not so much about having a secret affair, but rather about securing a "supportive connection" or friend in the background, just in case. Have you ever ended a relationship to be with your "back up" person? Did the relationship work? Did your partner ever suspect this person as a threat to your relationship?