Was just going to add this as a bug. Have the same problem.

As a suggestion, here is my fix for it:

Index: manage_proj_cat_copy.php

RCS file: /W/cvs/cvsroot/mantis/manage_proj_cat_copy.php,v
retrieving revision 1.1.1.1
diff -r1.1.1.1 manage_proj_cat_copy.php
13c13
< $result = category_get_all( $f_project_id );

$result = category_get_all( $f_new_project_id );
20,21c20,21
< if ( !is_duplicate_category( $f_new_project_id, $t_category ) ) {
< category_add( $f_new_project_id, $t_category );

  if ( !is_duplicate_category( $f_project_id, $t_category ) ) {
      category_add( $f_project_id, $t_category );

Glenn Henshaw (ghenshaw at altera dot com)

I fixed this... I think it was actually working already in CVS but wasn't because I was testing with register globals turned off and I haven't updated that page yet. Anyway, for completeness I made it actually work even without register globals and checked it in.