Google Play Duplicate Charge

I had a charge from Google Play to my T-mobile account for $19.99 which was erroneously sent through twice. Google Play shows the transaction as cancelled (it was actually a glitch in the game software that sent the charge through twice), and Google Play says that the second charge should never have posted to my t-mobile account so they cannot refund it. I paid the bill the month it was due, expecting a credit the next month. But I did not receive a credit. How to resolve?