[Satellite 6] Error during manifest import "Failed to import archive"

Issue

• Error importing new Satellite manifest file
• Attempting to import a manifest in Satellite 6 fails with the below errors in/var/log/foreman/production.log:

2016-10-24 12:50:04  [app] [E] Error during manifest import: {"displayMessage"=>"Failed to import archive", "requestUuid"=>"398feb37-f6bd-4709-998a-630f0e8f4a95"}
2016-10-24 12:50:04  [foreman-tasks/action] [E] Failed to import archive (Katello::Errors::CandlepinError)

Also the following errors may be witnessed in /var/log/candlepin/error.log:

2016-10-24 12:50:04,662 [thread=http-bio-8443-exec-7] [req=398feb37-f6bd-4709-998a-630f0e8f4a95, org=secondorg] ERROR org.hibernate.engine.jdbc.spi.SqlExceptionHelper - ERROR: duplicate key value violates unique constraint "cp_content_label_key"
  Detail: Key (label)=(rhel-7-server-containers) already exists.
2016-10-24 12:50:04,693 [thread=http-bio-8443-exec-7] [req=398feb37-f6bd-4709-998a-630f0e8f4a95, org=secondorg] ERROR org.candlepin.sync.Importer - Failed to import archive

• Downloaded new manifest file, but it will not upload to the Satellite. Stopped task has error message:

Runtime Error ERROR: duplicate key value violates unique constraint "cp_content_label_key"
  Detail: Key (label)=(rhel-7-server-containers) already exists. at org.postgresql.core.v3.QueryExecutorImpl.receiveErrorResponse:2,157