Thank you for writing.
I managed to replicate the described case. It is caused by the fact that the Columns collection of RadGridView is a serializible collection, and once you add a column to it, the column will get serialized. Then, when you start the grid, one column will be added from your code, and another from the serialized information in the InitializeComponent. You can easily handle this case, by only adding the column in the derived grid, if you are in design time. For the purpose however, you will have to use the Load event, instead of the constructor:
MyGrid : RadGridView
GridViewTextBoxColumn col1 =
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