Surprisingly answers to competitions are often shared by Compers! This usually isn't a major problem for some sites (unless it's an email entry) especially when the answer is perhaps in the blurb.
Other times, for example if a sponsor site has to be visited by the entrant to get an answer, it can cause a big problem. More complicated competitions like treasure hunts and quizzes can also suffer by entrants sharing answers.
So What Can You Do?
People will always share answers even for the most simplest of competitions - so the key is making sure it's difficult to share answers!
1. Random Questions
For example in our prize draws we use a script to pull out a product from a shopping feed and ask the user to tell us the price in order to enter the competition. This means (1) they have to visit the store and (2) as the products are random, so are the questions!
2. Treasure Hunts
Force entrants to take part in your treasure hunt by moving the objects daily or creating a specific treasure hunt for each entrant. This may involve more work for you though and it really depends on the technical skills you have access too. Be wary as people do like to post the locations - negating your hard work!
Again you can use random questions for different users, but this won't stop answer sharing. However you could include timed rounds, skill questions, picture questions and other elements to make the quiz more personalised and instant for each user. E.g the user answers 10 questions, enters their details and then they're given a timed random question in order to complete their one and only entry.
4. Personal Answers
One easy way around answer sharing is simply ask a question that requires a more personable answer. For example a fashion site may ask a user to select their favourite dress from a current collection or a movie site may want a favourite film starring Brad Pitt!
Essentially you have to be cunning when it comes to putting your competition together - or just accept that answers get shared.
If you find your answers have been posted online then usually a quick email to the site or forum requesting removal is enough to sort things out. Remember people are only trying to help and it's often good promotion for you.