assignment with same name should not be possible

How to reproduce:
1. Create new file
2. Create any category in Manage Category window
3. Open Manage Assignment window
4. Click Add
5. Change Contains field value to "a"
6. Repeat 4 & 5 ... -> duplicated rule names are possible (problem with loading them from saved file)
7. Click Close
8. Create account with name "a" in Manage Accounts window
9. Open Manage Assignment window
10. Click Add
11. Change Contains field value to "a"
12. Click on any old "a" rule in the list on the left -> selected rule name is changed to default name of previously created one

Patch attached.