In CRM 2013, while importing of solution, we faced the above in the Entity Ribbon of the Account Entity.
To resolve the same, we unzipped the customization file that we were importing and then opened the customizations.xml file in a xml editor and checked for the RibbonDiff part for the account entity.
We found duplicate entries in the HideCustomAction. See the screenshot below.
We just removed the duplicate entries and the re-imported the solution and it worked again
Hope this helps!