Error when importing an unmanaged solution

I'm trying to migrate some customizations from our development environment to the test environment. I have everything I need in a solution. When I import this into the test environment, it fails with the error;

An item with the same key has already been added.

In the error file that I download from the solution import screen, it's listed against the Form ItemType for the Incident entity. My solution contains a new custom form, it also has the definitions for the 2 default "Information" forms.

With verbose tracing, I can see the error in an xml string;