Try removing the following line from your web.config
<add assembly="System.Web.Extensions, Version=126.96.36.199, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
Clean the solution ( right click on the solution file and choose "Clean Solution"). Check your references from the GAC again. The MSDN says that the same file exists in two places, or that you forgot to change the version number.
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