According to Joseph Meehan :

The outlets have a ear that bridges the gap between the upper and lower
outlets, you need to remove all of these. If you are not sure, try getting a
good book at the DIY store, and if you are still unsure get a pro.

You break off only _one_ of the ears, on the hot side. The neutral is
shared.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.