Fun fact: "sama" is a cognate through Sanskrit with the English word "same".
So, this is an answer to "Thank you"? So in this context, could it mean "Same to you.", but not as an answer to "Have a nice day." ? We use "Same to you" more generally.
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)