Lots of compers get upset if somebody retweets their competition entry, fearing that it means they will be disqualified, so I'm going to take a look at what happens when you retweet something.
Let's say there are two compers, Alice and Barbara, who are both entering a competition run by Company. (See what I did there - I can call them A, B and C now!)
C Tweets "RT to win this smashing prize"
A sees the tweet and retweets it.
B sees A's retweet but instead of going to C's timeline and retweeting the original tweet, they retweet A's retweet.
Who sees what?
Well, if A used the RT button, her entry will show up in her timeline with C's avatar and two arrows underneath, along with a note "retweeted by A". If B also hits the retweet button, it will also show up in her own timeline, also with C's avatar, and with the note "retweeted by B". When C looks at their tweet to see who has retweeted it, both will show up. Both have entered the competition, A will probably never know that B has retweeted the tweet and all is well.
If A used the retweet button but B would rather enter by copying and pasting, she can copy the text of C's tweet into a new tweet. Once again A won't be affected. However B must be very careful of something - she MUST remember that as well as the text of the tweet, she needs to start the tweet with RT @C and then paste in the text - otherwise not only will C not see her entry, but new compers on Twitter who see B's tweet might think that B is running the competition and try to enter by retweeting it themselves!
If A has used the "old style" copy-and-paste way of retweeting, and B also copies and pastes her tweet, remembering to include the full text of the tweet including the part that says RT @C, then again all is well and A is unaffected.
Where it gets messy is when A has done a copy-and-paste tweet but B uses the retweet button. It will then show up in A's timeline as a tweet of hers that has been retweeted, and in B's timeline as a retweet of A's tweet.
In C's activity column it will show up as "RT @A RT @C RT to win this smashing prize" if it shows up at all so if C sees it at all, they won't see that B has entered, only that A has done so. That's because B, by using the retweet button, has told Twitter that it is A's message she is retweeting and not C's.
The message C sees won't be an exact retweet of their original tweet, so they may not count it as an entry to their competition, but if they do they will count it as a second entry for A and poor old B won't have entered at all. B's "entry" will have gone to A instead.
Picture it this way, if you are a comper who likes to do postcard entries from magazines. If A passed on her copy of C's magazine to you and you wanted to enter a competition in it, you wouldn't post your postcard to A would you? Well, retweeting A's tweet is the same as doing that!
So going back to the problem "Will we both be disqualified?" the answer is "probably not".
Most competitions on Twitter don't limit you to one entry per person. Those that do limit you will almost always allow your first entry even if somebody retweets it. However if somebody has retweeted your entry and you don't like the fact that they have done so, go back to your own message, the one they retweeted, and delete it. Their retweet will disappear too. Then all you need to do is tweet it again!
The moral of all this is that in certain cases it is OK to retweet another comper's entry, but to be absolutely certain that you are entered you need to go to the promoter's timeline and tweet from there.
Happy (re) tweeting!