No need to add "sự". Short and to the point should be:
Thân thiện luôn là một lợi thế.
I learned that "luôn" is often and "luôn luôn" is always. Is that wrong?
Thường or thường thường are often. Luôn and luôn luôn are always.
Nope, you're right, but I think luôn luôn is more of an emphasis on luôn
That is correct.
Friendliness will always be an advantage