This message appeared on plenty of end user devices across many of my clients when they try to synchronize their SharePoint Libraries using OneDrive sync client (Not OneDrive library itself though), no big deal, it was all about browser in our case (could differ in some case as well, I presume). The exact error is ‘You’re already syncing this account’. Open your OneDrive – Your Organization Name folder or sign in with a different account’
A common nature of the scenario was that, everyone got this error was using either Microsoft Edge or Internet Explorer. The immediate solution was using an alternative browser instead, we tried Chrome and it worked like charm.
When Chrome prompted options, choose: ‘Open URL : OneDrive client protocol’
And, you can now start syncing the library
Microsoft Cloud Infrastructure Technical Journal 