INT on a DP?

[qcumpire]

I've got interference. The retired runner interfered with the defensive player's opportunity to make a play on another runner (rule 8 section 7 P).

Since the player that committed interference is already out, you can not penalize him or her for the interference.

Whether the batter-runner would have been safe or out at first is irrelevent.

The penalty for the interference is applied to the runner closest to home, which in this case is the batter-runner.