The error message “Sorry, another account from your organization is already signed in on this computer.” appears when opening an Office document from SharePoint Online:
Follow these steps to solve the issue:
1. Open Excel on the desktop
2. In the top-right click Switch Account then sign out, wait for the next page to load then sign out again
3. When the sign-out is complete, reopen the document from SharePoint and you should be able to sign in successfully
