How did you re-download the app, from the main part of the store or the Purchased tab in the App Store app ?
You can contact iTunes support via this page : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption
I'm not 100% sure whether it would mean that you will be charged again if you re-download an app via the main part of the store (but it's possibly an explanation for being charged twice) - it's not something that I've ever tried, I always re-download via the Purchased tab where I know that I won't get charged.
If you have 2 accounts and you re-download the app via the 'wrong' account then you will be charged as all content is tied to the account that originally bought it
I'm download app at the right account. But there is something when I first time swap an apple id , finished sign in and immediatly go to the app that i
want to update then the button showing " the price " instead "INSTALL" or "UPDATE" i'm sure i have purchased this app
It might came from a slow responding from the store itself. And If you hit the button while it show the price it's mean
You got a twice charge. that what i understand.