I sent a little money to this address 19RTgakS1WZP172vZwn44HnJAKU8ZQcJxm and they said that this link was allready used by someone else. Now he is saying the bitcoins were sent to no one and that they're lost. Can I refund it somehow?
Depend on who sent to you (assume it a merchant) and what software they use. Some merchant apps may allow the address to be reused so they may still receive the funds.
A. They used a merchant app that only allows the address to be used once: Well I guess you're out of luck. If this happens, the bitcoin you sent you will basically be given to the merchant app developers (No such thing as sent to nobody unless the address is unclaimed which is unlikely in this situation) (Bitcoin is a one-way transaction, once sent, it is gone unless the target refunds them directly to you)
B. They use a merchant app that allows multiple uses of address or better yet bitcoin wallet: You can have them check the previous address and see if more funds are added to it and maybe do refund if requested.
If they control the private keys to that address, they have the ability to refund you. Most merchants generate a new address for each transaction, and even if you pay to the wrong address, they can derive the respective private keys to transfer those funds back to you.
Is the address you paid to from the same merchant?