Compiler Error CS1537

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The using alias 'alias' appeared previously in this namespace

You defined a symbol twice as an alias for a namespace. A symbol can only be defined once.

The following sample generates CS1537:

// CS1537.cs
namespace x
{
   using System;
   using Object = System.Object;
   using Object = System.Object;   // CS1537, delete this line to resolve
   using System = System;
}