It really depends on how the IW is set up. Sometimes you can enter the GP portion without playing the IW. Other times the IW portion is set up like a portal to enter for the GP, where it looks like you have to play, get a winning or losing screen, and then are entered. You often get a message like, "While you didn't win this time, you are still entered to win the grand prize. Come back and try again tomorrow." When sweeps are set up like that, I have the same question as you, if you did win, how can you continue to enter?
If I am lucky enough to win an instant, I check the rules to see if I can still play. Sometimes, you just can't tell. If that is the case, I wouldn't want to win the IW again, and not get the prize, and have it forfeited when someone else could have won it. A few times, when I won an instant which was super hard to win, I entered a few more times because the GP was really good (but never won it anyway), figuring I had such a slim chance of winning the instant again.