Duplicate triggers can be imported from XML

Steps to reproduce:

1. Create a host with an item and a trigger.
2. Export the host.
3. Rename or delete the host.
4. Open export file in a code editor.
5. Make a copy of the trigger so that there are two equal records under <triggers>.
6. Save the file and import it.
7. Navigate to the list of triggers of the host.

Result:
There are 2 triggers with the same name and expression.
Expected:
It should not be possible to import duplicate triggers from XML.