Problem description: If a configuration import file contains 2 (or more) items with the same key under the same host, such configuration file is successfully imported and the last item with the duplicate key overwrites the previous items with such key. A duplicate key validation error is expected in this case (same as it currently works for item prototypes).

Steps to reproduce:

1. Create a host with an item
2. Export this host
3. Delete or rename the Host that was just exported
4. In the obtained configuration file copy-paste the item in <items> section and change the name of the new item entry (for example from Item_1 to Item_2).
5. Navigate to Configuration => Hosts and import the previously modified configuration file.

Result:
The configuration file is successfully imported. The imported host contains only 1 of the items with duplicate keys (last in the file).

Expected:
A duplicate key validation error should be returned when attempting to import the configuration file.