This is an enhancement request based on my own and others experience and also documented in Jochen Topf's issue tracker concerning the multipolygon fixing effort (see https://github.com/osmlab/fixing-polygons-in-osm/issues/29).
The problem is that it is to easy to accidentally copy relations, and this happens especially with multipolygon relations where novice JOSM users attempt to add new members. As a consequence, multiple stacked multipolygons are created, with sometimes more than a dozen copies having different counts of member ways. This is undesirable and difficult to clean up.
What likely happens is that instead of choosing "Modify relation", users accidentally click "Copy relation". This buttons sits right next to the button for modifying a relation. This means that if people accidentally hit it while intending to modify an existing relation, they may create a new relation without even knowing it, as it will have all the members and properties of the original and thus appear as the original to be modified. When they subsequently upload the changes, they will have created a copy instead of having modified the original.
It would be good if JOSM somehow warned for this type of (accidental) misusage in the interface.
I therefor propose an absolute minimal change to JOSM that is likely to minimize to chance of this happening accidentally: add a blocking modal dialog with the question
"Are you sure you want to copy an existing relation? Yes/No".
to the "Copy relation" button before opening the relation editor. This will make novice users aware of the fact they are copying an existing relation instead of modifying it.
Maybe restrict to
type=multipolygon?