CS1703: An assembly with the same identity has already been imported. Try removing one of the duplicate references.

Compiler Error Message: CS1703: An assembly with the same identity 'System.ComponentModel.DataAnnotations, Version=99.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' has already been imported. Try removing one of the duplicate references.

I can't find the duplicate reference anywhere, have looked in property windows -> references , and its not listed anywhere else.

Any ideas?