[Hackathon] Duplicate trigger names should not be allowed in the same dialog

[v-kydela]

Describe the bug

It's unclear what purpose trigger names serve when the name isn't used to identify an intent, but I would think the name should at least differentiate the trigger from other triggers. They can only serve that purpose if uniqueness is enforced.

To Reproduce

Steps to reproduce the behavior:

1. Add a trigger to a dialog
2. Add another trigger to that dialog with the same name
3. See lack of error

Expected behavior

There should be some kind of message that says the name is already taken.

Screenshots

[bug]

[sangwoohaan]

Will be addressed in #778