It happens all the time. We tell a story to our friends only to realize (probably as we're just reaching the end) that we've told this story to this same group of people before. It may be a good story, but no one needs to hear it over and over and over again.
I come upon this every time I go through first drafts - whether it's of a novel or a game. I've given the same clue more than once. Sometimes, it's way more than once. What's amusing is oftentimes it's not even that vital a clue - just something that stuck in my head.
But repetition sneaks onto our pages in many guises. I've found I have love affairs with certain words as I'm writing that first draft. This is when the 'search and replace' function gets used. Thank heaven for word-processing functions!
Another mask repetition wears is when you discover your characters tending to react to situations in the same way. There can be a surplus of eyes widening or shoulders shrugging or throat clearing. High nervous laughter can also litter your pages. Be aware. I've found knowing my characters can help avoid this pitfall - if I think about one characters and how he/she would react in character, it's usually unique unto them.
How do you avoid repetition?