Yes, you can enter as many projects into a contest as you like. However, one project can not be entered into more than three contests, so you've got to choose wisely!
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Depends on the contest. Most allow more than two entries.
Yes but -- do you really want to compete against yourself? If you have 2 really good instructables you might want to submit the best and see if you can improve the other for another chance later in a different contest. Or if they are a similar topic merge them together into an even better single one.
yes most of them say you can but you'll get only one prise. I think you should read all terms and condition it is just at right side column of any contest page