You can always check this by looking at your visa, and it saying single entry or multiple entry. If it says single entry, this means that you may only use your visa once, and if you try to use it any more after that, they will stop you in customs and send you back home. Please do understand that if this happens, there is little we can do, and you are at the mercy of customs. To prevent this, you should always check your visa and make sure that it is a multiple entry visa and if it is a single, that you have never used it before. If you have anymore questions. please submit a ticket.