Do you have to re-purchase Minecraft pocket edition again if you've already bought it on another device. Like if you bought it on an android phone and you're getting a new android phone do you have to re-purchase. Or will it just transfer with your Google account
Purchases are specific to your account, not your device. No, you do not need to re-purchase Minecraft PE.
If you switch to another mobile OS like iOS, you will of course need to purchase it again, though.