mail.app: invisible gmail account
i've added a gmail account to mail.app and it's showing up in the Internet Accounts pane of my System Prefs (with the Mail, Contacts, Calendars, and Messages boxes checked), but there's no sign of the account anywhere in mail.app - no associated mailboxes (including inbox), it doesn't appear in mail.app's accounts list, and mail.app isn't receiving mail sent to that address (or if it is, it isn't displaying it). yet when i try again to add the account, i get an error saying it's a duplicate.
what the @%$#??
MacBook Pro with Retina display, OS X Yosemite (10.10.3), 16 GB RAM & a bunch of other specs...