No you don’t! When downloading Danger Zone onto an additional iOS device,or re-downloading it on the original one, you will not be charged again. The downloading procedure can be a little confusing. First you will be asked if you wish to purchase the item. Tap "Yes", and you will be given the price etc. But once you type in your password in the iTunes Store, you will be notified that you have already purchased Danger Zone. You will then be asked if you wish to download it again. Tap "Yes", and you will get the feature for the additional iOS device! Of course you must be using the same iTunes Store account (ID) that you originally used when purchasing Danger Zone.
Please note that the HD version and the regular version are separate games, so the In-App Purchase of Danger Zone cannot be transferred from one to the other.