I recently dipped my toes in Nethack waters and so far I've been letting Nethack pick the character for me. I keep getting either archeologist or a cavewoman though for about five games now. I'm wondering, is it supposed to be that you get a random character and I've been really (un)lucky to get the same two types for my games?
The almighty Random Number God decides your fate. Have you been praying to him? If not, no wonder you have been getting the same characters over and over!
In all seriousness, it's almost completely random. To be exact, pseudo-random due to the nature of computers, but the distribution should be almost even.
If I were you, if you get the same character for a sixth time, then just ask for a new character. Maybe the RNG will have mercy.