Twins in Middle-earth seem to be a common occurance. It is mainly noticed among Elves and Half-elves, but some have been recorded among Men and Dwarves. It is interesting to note that many of these twins are ancestors of, or siblings are ancestors of, Elladan and Elrohir, who were themselves twins.
Elven and Half-elven Twins
As has been said before, twins seem to be common among Elves unlike the human standard of one in sixty. Elven twins:
Twins run through the female's genes. That means if the father had ancestors that were twins, but the female doesn't, the chances of having twins is very slim. However, if their daughter has children her father's genes affect her chance of having twins.
--The Lord Rhys 16:08, 10 November 2006 (EST)