With array you'd have to compare each new number to all of the existing numbers if the new one is already there. You could give the user a message and let them try again. So if they were supposed to enter 10 numbers it might take them 15 tries to get the idea that they have to be unique. Is that how you'd picture the program working?
If you're allowed to explore beyond array, look at the JavaDoc for Collection, List and Set. There is one there you'll like.