Bulk REST API v2 Activity Data Export "Primary Key"?

After reading the documentation, I expected that ActivityId would be the primary key when exporting activity data -- e.g. that there would be no duplicate ActivityIds in the items data returned by the export. However, I am seeing exported data where I have multiple activity datums that are identical except they have different CampgainIds

1. Is ActivityId,CampaignId the primary key for activity data? If not, what is?
2. When will activity data with the same ActivityId have different fields (excluding CampaignId)?

Thanks!

Christopher Campbell