Created Unassigned: 'Model.Context.tt already exists' -- Designer attempts to add template when one has already been added. [1176]

1. Add new empty model
2. Add code generation item. (ef6 dbcontext template)
3. Update model from database or generate DDL

Error shown:

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Microsoft Visual Studio
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An exception of type 'System.Runtime.InteropServices.COMException' occurred while attempting to generate the database script from the model. The exception message is: 'A file or folder with the name 'Model1.Context.tt' already exists. Please give a unique name to the item you are adding, or delete the existing item first.'.
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OK
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