multiple heads containing the same change sets: how to get rid of them?

[solo turn]

hi,

why is it possible that mercurial creats multiple heads on pulling in
changes in the following way, changeset history upwards down:

tip: 4 - 3 - 2 - 1
other head: 2 - 1

then, on a merge, mercurial tries to merge in the difference from 4 to
2, so basically somehow putting in the old staff from 2.

what is the best way to get rid of the head which does not contain a
changeset not already in tip?

-solo