Error accessing values in DynamicClass - fails on second attempt

[glittle]

This is broken in version 1.2.20. Version 1.2.19 is working.

1. Description

With this statement:

 var result = DataContext.Users
              .Where(u => u.Id == id)
              .Select($"new {{{"Name,Email"}}}") 
              .ToDynamicList()
              .FirstOrDefault();

The result variable has two properties: Name and Email.

On first access to a property, it works.
On second access to any property it fails with the exception shown.

For example:

var a = result["Name"]; // okay
var b = result["Name"]; // crashes with exception

2. Exception

'result["Name"]' threw an exception of type 'System.ArgumentException'
    Data: {System.Collections.ListDictionaryInternal}
    HResult: -2147024809
    HelpLink: null
    InnerException: null
    Message: "An item with the same key has already been added."
    ParamName: null
    Source: "mscorlib"
    StackTrace: 
		"  at System.ThrowHelper.ThrowArgumentException(ExceptionResource resource)
		   at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
		   at System.Linq.Dynamic.Core.DynamicClass.get_Properties() in C:\\Dev\\GitHub\\System.Linq.Dynamic.Core\\src\\System.Linq.Dynamic.Core\\DynamicClass.cs:line 36
		   at CallSite.Target(Closure , CallSite , Object , String )
		   at System.Dynamic.UpdateDelegates.UpdateAndExecute2[T0,T1,TRet](CallSite site, T0 arg0, T1 arg1)"
    TargetSite: {Void ThrowArgumentException(System.ExceptionResource)}

[StefH]

Maybe also related to #634 ?