I accidentally sent bitcoins twice to the same address
It is not clear what exactly happened. 


If you've made two subsequent transactions to the same address that should not be a problem. Addresses can be used more than once to receive money.
If you created two transactions that spend the same transaction outputs, there is no problem, because only one of them can confirm.
If your transaction simply didn't get confirmed yet, there is little to no problem: Transactions either have confirmed or have not. Since both wallets are your own, the money is still in your sending wallet until it is in your receiving wallet. If you wish to get help about fixing a stuck unconfirmed transaction please refer to: Why is my transaction not getting confirmed and what can I do about it?


If your question is not covered by these three scenarios, please edit your post to clarify what you're asking.