I was charged for a redownload of an app I previously paid for

Did you change your Apple ID? Is the app in your purchases history?

Purchase History Review.  

Normally, all sales are considered final, but you can ask for a refund.

If you have received a confirmation email, click the Report a problem link at the bottom.

Purchases problem.      

How long refunds from the App Store or other Apple services take.  

That is why you were charged.