Contacts in two Categories are Combining to create a third Categor

I have just started having a problem on two of my users computers. They are
both running Outlook 2003. One is in cached mode, one is not.

The users are having their contacts that are members of two categories
combine into a third category.

Example: Mark Smith is a contact that is a member of the category
"Business" and "Competition" If you look at the contact details for Mark
Smith you will see "Business, Competition". If you reboot the computer and
look up Mark Smith he will now be a member of the category
"Business,Competition". The space goes missing and now the two categorys
have become a third category "Business,Competition".

If you go back through and put the space between Business and Competition,
the contact is categorized properly. After the user reboots, it returns to
the broken state.

This has only started happening in the past two weeks. Has anyone else had
this problem? How did you fix it?

Thank You,