Knock knock. Who's there? Citation needed.

Citation needed who? This joke requires a source to be funny.

Knock knock. Who's there? Dough.

Dough who? Dough-n't worry, I'm just here for the bread!

Knock knock. Who's there? That's what I'm asking.

That's what I'm asking who? That's what I'm asking who wants to know who's on the other side of this door!

Knock knock. Who's there? Ivan.

Ivan who? Ivan telling you to open this door for ages, my knuckles are practically knocking themselves!