If your TFS client cache is corrupted, you might see this error:

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Microsoft Visual Studio
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Error

An item with the same key has already been added.
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OK  
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Here's a solution, run the following to clean the cache:

SET AppDataTF=%USERPROFILE%\Local Settings\Application Data\Microsoft\Team Foundation
SET AppDataVS=%APPDATA%\Microsoft\VisualStudio
IF EXIST "%AppDataTF%\1.0\Cache" rd /s /q "%AppDataTF%\1.0\Cache" > NUL
IF EXIST "%AppDataTF%\2.0\Cache" rd /s /q "%AppDataTF%\2.0\Cache" > NUL
IF EXIST "%AppDataVS%\8.0\Team Explorer" rd /s /q "%AppDataVS%\8.0\Team Explorer" > NUL
IF EXIST "%AppDataVS%\9.0\Team Explorer" rd /s /q "%AppDataVS%\9.0\Team Explorer" > NUL