The SketchUp UI seems to enforce that only unique names can be given (replace existing definition with same name by this new definition or abort).
The API doesn’t explicitely state that the name must be a unique identifier, but it mentions a GUID.
But when you rename a definition to an existing name, it doesn’t apply the new name. Instead the API docs say it returns “the name assigned to the component definition if successful”, but actually it always returns the passed in parameter because that’s Ruby’s default behavior for assignments.
So there is a documentation bug and a component definition import bug. Interesting would be whether the bug occurs also when importing the definitions as skp file, or only when importing from 3D Warehouse.