It's also important to know what the sponsors mean with different sweepstakes entry restrictions. If you're not sure what a term means, check Understanding Sweepstakes Entry Restrictions for more information.
However, everybody makes mistakes sometimes, and I don't think that there are any sweepers who haven't accidentally entered more often than the sweepstakes frequency allows. Perhaps you forgot you had entered the sweepstake, or the deadline was extended and you thought that it was a different contest, or maybe you simply clicked the "submit" button twice by accident.
So if you make a mistake and enter a sweepstake more often than the rules allow, will you be disqualified and lose any chance to win? The answer is: sometimes.
In general, sweepstakes sponsors have two ways of handling duplicate entries: they either disqualify all entries made by the offender, or they only count the first entry made. (If it's a daily sweepstake, the sponsor can choose to disqualify the entrant from the entire sweepstake, disqualify all entries made that day, or discard all entries made after the first). If the sponsor chooses to discard only the entries made over the limit stated in the rules, then you still have a chance to win.
How can you tell which method the sweepstakes sponsor uses? In most cases, it will be stated in the rules. If the sweepstakes rules don't mention how duplicate entries are handled, you need to cross your fingers or contact the sponsor directly to ask.