Mega Kangaskhan's ability Parental Bond makes him use the same attack twice, with reduced power the second time. Some attacks only hit once, like Explosion. Note that this only applies to attacks, so you can't use a status move twice. So while you can't use Protect + Focus Punch in the same turn, you can use Power-Up Punch twice in one turn, giving you a +2 in Attack (a common strategy). In addition, Seismic Toss will hit twice, but will not have its damage reduced, so assuming you're level 100, you get fixed 200 damage, allowing you to 2HKO a lot of Pokémon after some prior damage from Stealth Rock/Fake Out/etc.
Source(s): Knowledge and https://bulbapedia.bulbagarden.net/wiki/Parental_Bond_(Ability)