I checked one of the triggers I'm using and noticed a slight variance than the above screenshot...which I probably noticed after trying to edit features!
The sample below should take care of handling features were a sequence has already been applied. The add (a) delta table for this test feature class has the trigger below applied to it, which is set to update the ID field on an insert/create IF the ID field is NULL. You can see on subsequent edits of the feature, the 'ID' field remains the same value because the field is not NULL, and thus the trigger is skipped.
Modified trigger to account for already-populated sequence value. The 'IF' statement on line 27 will check the inserted table to see if the sequence field (ID in this example) is already populated. If it's not, the next value within the sequence will be grabbed and populated into the 'ID' field as shown on line 35.
I blocked out our database and schema name. Simply replace with your values.
Add (a) delta table showing edits. ID field remains as-is on subsequent edits because it is already populated.