From what my girlfriend who is from Medan states, she said saying sama-sama is basically saying "same" or "back at you" "same to you". Not Your welcome. so basically sama-sama could be used as a response to someone saying thank you to mean your welcome. but in another example it could be used below.
Friend: Aku suka jeruk (i like oranges) You: Sama-Sama (same)
Because that is a different meaning of the English word "welcome". By itself, "welcome" means something like, "I am glad you are here". In Indonesian, I think that is, "Selamat datang". "You're welcome" is what Americans say in reply to someone who has thanked them. It means, "No need to thank me". That meaning is "Sama-sama" in Indonesian.