importer bugs?

Ok this is driving me crazy. I had to import some posts from a site to a new site / test site. Basically I was going to merge the two.

It seems that wordpress on importing the new posts, shifts all my existing posts OUT of their current categories into just top level categories?

Is this a bug with the importer? Is the importer just writing over existing posts? I have no idea what’s going on, except that it doesn’t make sense for wordpress too:

a) over write posts that are already existing with imported posts. (example the importer will tell me “oh that posts already exists” but then that already existing post has been shifted somewhere else).

and

b) shift everything into top level categories.

Highly frustrating. Any ideas?

Thanks