TFS error during adding new file in checked out folder

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VS2008

Visual-Studio

Hi,

I'm tring to add a new DLL inside a folder after checking it out (folder).
After copying that DLL in the working folder and while checking the folder, I get the below error, please let me know the solution:

All of the changes were either unmodified files or locks. The changes have been undone by the server.

I'm using Visual Studio Team System 2008.

Thanks
RajRam