This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
There's no implementation inheritance, so it doesn't matter which interface you're referring to--you can only implement one method with a given signature. No matter which interface your object is currently acting like it'll call the same implementation--no diamond problem. The diamond problem refers to implementation disambiguation.