You'll need a clause in your agreement with your designed that protects against prior use. The only way to guarantee your logo (in the U.S. anyway) is to trademark it, and then more importantly, to protect it. It CAN be a costly process.
Trademark it for the major market(s) you will be operate in. There are seperate registers for each country, however EU got a shared one. It may take a while to get registered. It took me 7 months last time I registered in my national register and the cost where USD 420,-.