I have already paid for app on one of my devices, but Google Play is asking me to pay for it again on my other device.
If your purchase was recent (less than a day ago), then allow some time for Google Play to synchronize purchase information. It may take up to a day before the purchase information reaches your second device. You could also try rebooting your device, which sometimes speeds up this process.
All extra purchases should be automatically refunded to your card.
Also, check to make sure that both of your devices are linked to the same Google account. You can check this in the Accounts area of your device's settings.