R: sample function repeats same results

I ran into something very weird with sample(). If I run the following line 5 times at the start of a session (in either RStudio or R), I would get the following results.

sample(letters,5,replace=TRUE)
[1] "b" "y" "d" "p" "n"
[1] "v" "n" "i" "s" "s"
[1] "d" "q" "a" "m" "x"
[1] "w" "s" "u" "h" "e"
[1] "b" "y" "g" "s" "e"

But if I restart the console and run it 5 times at the beginning of a new session, I would somehow get the same results -- every time. Is sample() (which I believe uses Mersenne Twister by default) supposed to do this? What should I do instead to get results that don't actually repeat?