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The field SerialNumber already exists for some group objects (e.g. "CI.AccessPoint").
It's a known issue with shared fields which is fixed in 2016.1.1. Now, if field names are overlapping, you are being asked if you want to use this shared field.
As a workaround you can (more precise: have to) create your own field on the CI object and use this one instead.
As soon as you have upgraded to HEAT 2016.1.1 or 2016.2 you can revert this change.
That's how I did it.