We have got three projects with nearly the same categories. Projects two and three were both created out of the first one weeks after the first one was created, so we had to move some issues from project one to projects two and three. Before we did that we created the categories with nearly the same names as in project one.
After moving the issues the statistics (especially grouped by categories) on the summary-page were messy. Categories with the same name were shown several times. And selecting one of the numbers shown in the table leads to a page with all the issues for that category listed.

I think, the problem is, that after moving the issue to the new project the category-id is not updated. It's still the category-id from the old project. But the categories are assigned to a project by mapping the corresponding id's in the category-table, right!? So after moving an issue it has to be the corresponding category-id from the new project (searching the id by name) and if there is no category-id mapping the name the issue should be assigned to the default-category with the id 0.