Looks like others are having this problem but each case is a little different.
A few weeks ago I submitted a new extension for the first time. The extension is unlisted and does not have an update URL because it ships within my OS X native application. It was approved immediately.
Today I submitted an update, with the version number bumped by +1 but the same identifier (id) in the install.rdf. I plan to ship the update within the next version of the same OS X native application. Result:
Your add-on failed validation with 1 error. Duplicate add-on ID found.
What might be wrong? Thank you!