adding a trigger as a dependency that already is added should either produce warning, or, ideally, this trigger should not appear in the list to select at all. currently it is allowed to select such a trigger, but it mysteriously is skipped.

additionally, it is possible to add trigger as depending on itself. it only produces error upon saving, not upon adding.

ideally such triggers would not appear in lists to select at all, or they would appear, but greyed out