When I growed up melon seeds, it appears to seems 2 stem in the same place.
Then I break it and sometimes It drops one more seed.
Also occurs with pumkin seeds too
STEPS TO DO:
Simply plant a melon/pumkin seed
Grow it with bone meal
Note that the stem seems to be 2 in same place
Break the stem and you got 2 (or more in case of pumkin) seeds
Also I uploaded a video with this bug and screenshots I make before video
PS: Apologize my English