The comment on this issue was not fixed:
Furthermore, the change event is fired twice when you select an item with the mouse.
1. Open the Combobox.
2. Select an item with the mouse: First change event is fired.
3. Click outside of the Combobox: Second change event is fired.
This was also not present in the 2017 R3 SP2 version.
This same exact process happens with Dropdownlists too.
We can replicate this when using the ASP.NET MVC helpers, but cannot replicate this in the dojo.