When sending a signed a/o encrypted mail, I'm always asked if the correct key was chosen. I don't know if this happens because the mail-account is the second user id, or because the user id has capital letters and the account is transmitted as small letters only.
In any case, it's unnecessary. Also, there is no other "own" key at all, so asking to choose another makes no sense.
Gpg4win 3.0.3 GpgOL on Outlook 2010 32-bit on Windows 7 64-bit