Steps to reproduce:

1. Navigate to Hosts/Templates list page
2. Select few hosts/templates
3. Navigate to the bottom of the page and clack Mass Update
4. Select Value mapping tab
5. Check Value mapping checkbox, click on Add from template or Add from host
6. Select any Value map
7. Click Cancel
8. Do not refresh page. Repeat steps 4-6

Result:
Observe, that for each repeat of steps 4-6, the number of added Value Maps are going to rise.

Expected:
The Value Map should not be duplicating.