Here's an except from the US official rules just to clear up any confusion about only being able to enter once:
"Limit one (1) Play per Game Code per name/e-mail address per day, regardless of the method you obtained the Game Code. For the purpose of this Game, a ?day? shall be defined as 12:00:00 a.m. EST to the following 11:59:59 p.m. EST except for the first and last day of this Game.."
One entry per day. Not one and done. I just entered another a code for today and didn't win. The screen when I go back now says "Welcome back. You have already entered today." Also, the rules define a day as 12am-11:59:59PM which to me means that when midnight strikes it's another day and another code can be entered. That's not the case. I entered yesterday around 3:30pm and won $5 play store credit. I tried to enter today around 3pm and was told I had already entered today. Waited till tonight and was able to enter. So, it's 24hrs after you play a code.