Steps to reproduce:

1. On one host or template create 2 triggers: Trigger 1 and Trigger 2
2. Edit Trigger 1, click Dependencies tab
3. Click Add
4. Observe the overlay:
   
   The list contains Trigger1, so it looks like trigger can be dependent on itself.

Expected:
Although circular linkage is checked on Trigger saving form and error will be shown anyway, good UIX practice would be just filter subject trigger off from the list.