That's an interesting reply.

I should have made a subtle distinction between mailing lists and re-directions. Although the latter may be viewed as a special case of the former, you can't delete someone's re-direction on the email server just because it causes conflicts elsewhere.

Suppose we have one broadcast submitted by Arnold:

A --> b

A --> c

and two re-directions submitted individually by Bob and Calvin (which you can't delete because these people don't want ANY emails):

b --> A

c --> b

Althoug A is a valid email address, it's meant to represent a mailing list (eg. customers@myCompany.com) while b, c are real people (eg. Bob@myCompany.com).

There are no duplicates in this list, but both circularity and redundancy exist.

We could remove the second re-direction, but then Calvin would start getting emails.

If we expand mailing lists on the right hand side, we would get:

A --> b

A --> c

b --> b

b --> c

c --> b

Eliminating self re-directions gets us:

A --> b

A --> c

b --> c

c --> b

Since we can't eliminate the last re-direction submitted individually by Calvin, how do we get rid of the duplicates on the right hand side? We could eliminate b --> c, but then Bob's now getting emails.

Already my head's spinning.

You may be correct in your approach, but it looks a bit messy (to me, anyway).

That's why I viewed these things as graphs, where mailing lists and re-directions are the same thing.