You what?

I'd hazard a guess that you know at least one person who responds to everything you say with "what?", "huh?" or "eh?". In each case they heard you perfectly well the first time, though still insist you repeat yourself before commenting or answering a question. They know it and you know it.
I could cheerfully strangle them to death when I start to repeat myself and become tongue-tied, stutter or get my words back to front because I'm so preoccupied with mulling over how irritating it is to be repeating myself at all. More often than not after you've echoed yourself, the oaf you're speaking to will reply "oh" or "mmm" and then go on their way, or even worse, they'll ask a handful of irrelevant questions and then reply with "oh" or "mmm" and go on their way.

My theory is that they do this to buy themselves some extra time to respond. They seem to think that if they comment right after you've repeated yourself they'll appear sharp or 'on the ball', that they can think on their feet without 'umming' and 'errring'. I'd take half a dozen 'umms' and 'errs' over a "what?" every time. A few seconds of silence would do just as well; this isn't radio, we don't have to revere the 'dead air' taboo.

If you get riled, insist "you heard me the first time" and refuse to repeat yourself, they become defensive, reasserting that they definitely hadn't. They'll actually go so far as to appear insulted at the accusation. Well they're not likely to admit, "yes you're right, I did hear you, but can't shake this ridiculous habit of feigning deafness".

Try this instead: whenever someone responds to something you've said with "what?", pretend you haven't heard them and wait a few seconds. It's very likely they'll answer your question or pick up the conversation as though no 'whating' had occurred. They'll assume their "what?" hasn't registered and so won't feel obliged to wait for the repetition, or get caught up in a "yes, you BLOODY WELL did hear me", "no I didn't" battle of wills.

Turning the tables to wind them up can be more fun still. Pick a topic that you know is close to their heart and broach it with them. For instance, if you know they're waiting anxiously for an important call from their partner or potential boss-to-be (if they've been for a job interview recently), try opening with "so-and-so called earlier". When they reply with "huh?" as though they'd just woken up from a hundred-year slumber, hastily glance at your watch and tell them you're late for a meeting of some sort. As you dash out of the door, turn back and shout, "I'll tell you all about it later".