To copy to an existing document, you must specify the current revision string for the target document, using the rev parameter to the Destination HTTP Header string. For example:

COPY http://couchdb:5984/recipes/_design/recipes
Content-Type: application/json
Destination: recipes/_design/recipelist?rev=1-9c65296036141e575d32ba9c034dd3ee

The return value will be the new revision of the copied document:

{
   "id" : "recipes/_design/recipes"
   "rev" : "2-55b6a1b251902a2c249b667dab1c6692",
}