Only include test data in flattened apex 1 time

Previously, if a data source was included in multiple APEX, it would
appear multiple times; however, this caused errors for overriding
commands for targets

Test: apex_test.go
Test: m nothing with change I54d92eca88fc04c949209d490e838d0a92ce8f87
Bug: 155820504
Change-Id: I98f04e0fd9fa3238f2bb0e5da3a86fc0797c38ba