Hello Mark,
I just checked and the error message you're getting does appear to be
accurate (albeit inconvenient). There are actually two entries for
your GMail address in our Sandbox's user account data store. There are
a handful of other addresses with duplicates, though the majority of
email addresses have only one entry.
I've opened a bug with the engineering team to see if they can figure
out how the duplicate entries might have been created. This behavior
does appear to be limited only to the Sandbox environment.
I don't have the ability to delete your extra entry and since there
shouldn't be any data in your Sandbox client accounts following the
refresh, I think the quickest thing would be for you to create a new
@
gmail.com address and use that for Sandbox access. The extra entry
for your account should automatically be cleared up next month when
the Sandbox environment is reset, and you can go back to using your
original address at that point if you'd prefer. Apologies for the
inconvenience in the meantime.
Cheers,
-Jeff Posnick, AdWords API Team