Problem description: It is possible to obtain value mappings with duplicate names after changing the name of a value mapping to a name that already exists for another value mapping under the same host/template. Such configuration will not be saved due to existing API validation, but validation of frontend side doesnt allow to create value mappings with duplicate names, therefore, also the update operation shouldn't.

Steps to reproduce:

1. Open configuration of any host template
2. Add 2 value mappings with different names
3. open the configuration of the 1st value mapping and change its name to the name of the 2nd value mapping and submit the "Value mapping" overlay dialogue.

Result: Overlay dialogue was closed. ** Value mapping with duplicate name added to the list of host value mappings. 

Expected: Overlay dialogue should not be closed, an error should be shown, stating that a value mapping with such name already is present on the host/template.

Example: