When Rebuild this website warning this error ?

When I Rebuild the error below, how to fix and fix errors ?

Warning 1 The type 'webapp4U.UI.Controls.Controls_MenuLeft' in 'c:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\website2010\e7f1df91\85da5ad5\App_Web_2oy154p1.0.cs' conflicts with the imported type 'webapp4U.UI.Controls.Controls_MenuLeft' in 'c:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\website2010\e7f1df91\85da5ad5\App_Web_kgvuexfr.dll'. Using the type defined in 'c:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\website2010\e7f1df91\85da5ad5\App_Web_2oy154p1.0.cs'. c:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\website2010\e7f1df91\85da5ad5\App_Web_2oy154p1.0.cs