Issue

After Importing a Schema within BizTalk as a reference in the Collection under the Properties of the Main Schema node, I received some frustrating error:

This Operation Could Not Be Completed. Global Element Has Already Been Declared

Root Cause

This proved to be some Visual Studio contention issue. 

Solution

Close the .xsd file and reopen. This should allow you to create a Child Node, and select the proper (Reference) type, as expected.