When trying to sync OneDrive using the new sync client with Windows 7 and Internet Explorer (IE), users receive message "You're already syncing this account".
This issue is seen when syncing SharePoint team sites using the newer version of OneDrive.exe (17.3.6743.1212 or higher).
Internet Explorer on Windows 7 handles protocols differently from newer versions of Windows (8/10), and other browsers (Chrome/Firefox).
Sync the SharePoint team site using a non-IE browser i.e. Chrome or Firefox.
Microsoft's product group is aware of issue and is currently working on a fix.