AssertionError: Found more than one key for address

I cannot yet give a completely reproducible path for this (too many attempts/errors with the same account), but while testing an account with the current async-api branches, and after several errors, I got this keymanager AssertionError:

Traceback (most recent call last):
  File "/home/kali/Virtualenvs/leap-bitmask/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 577, in _runCallbacks
    current.result = callback(current.result, *args, **kw)
  File "/home/kali/leap/keymanager/src/leap/keymanager/openpgp.py", line 538, in get_key_from_active_doc
    'Found more than one key for address %s!' % (address,))
  File "/home/kali/leap/leap_common/src/leap/common/check.py", line 48, in leap_assert
    assert condition, message
AssertionError: Found more than one key for address test_imap_004@cdev.bitmask.net!
Traceback (most recent call last):
  File "/home/kali/Virtualenvs/leap-bitmask/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 577, in _runCallbacks
    current.result = callback(current.result, *args, **kw)
  File "/home/kali/leap/keymanager/src/leap/keymanager/openpgp.py", line 538, in get_key_from_active_doc
    'Found more than one key for address %s!' % (address,))
  File "/home/kali/leap/leap_common/src/leap/common/check.py", line 48, in leap_assert
    assert condition, message
AssertionError: Found more than one key for address test_imap_004@cdev.bitmask.net!

Which makes me think that the key generation is not properly checking for completion of the sync. I think we need to check the conditions in which the client backend generates a new key.

(from redmine: created on 2015-01-20, closed on 2016-07-05, relates #6536 (closed), relates #6691 (closed), relates #8059 (closed))