I'm pretty sure you're right kettle1 about it being a FAM. The reply I got back from Delta makes no mention of duplicate seats.
"We understand the importance of providing pre-assigned seats for our
customers and try to honor all seat selection requests. Seats are
reserved on a first-come, first-served basis up to 331 days prior to
departure. However, seat assignments are never guaranteed. We understand
that our Diamond Medallion Members are more likely, because of their
travel frequency, to experience reassigning of seats due to aircraft
changes or operational constraints. In situations such as this,
customers may need to arrange new seating with our agents at the gate
prior to boarding.
In the meantime, reservations are always changing and seats may open
back up. You may view or change assignments at any time to your
reservation online by visiting delta.com.
Our goal is to provide the highest possible standard of service in all
areas of our operation, and I know there is nothing I can say to erase
your disappointment with our service, but please know our apologies are
most sincere and we regret any inconvenience."