From 1814 to 2003, Danish parish registers were kept in duplicate, but the actual procedure was not the same from time to time and place to place. Sometimes, the minister and the sacristan met regularly and then the sacristan copied the entries from the minister's book into the sacristan's book. Other times, both the minister and the sacristan kept their books simultaneously and compared them regularly and made corrections if needed. I would say that by the first procedure, the sacristan's book is a derivative record, but by the second procedure, the sacristan's book is an original record. Anyhow, the recording procedure in a parish at a specific time was not recorded, so I don't know whether they did it one way or the other. Do I then treat all the sacristan's books as derivatives?
If possible, I always consult digital images of both books, because differences occur. However, for a lot of parishes, the minister's books have not been digitized, but only the sacristan's books. For preservation reasons, the original records cannot be accessed, meaning in many cases I only have access to the (presumed) derivative record.
Lene D. Kottal