I'm considering implementing a WFS client that, each time the map's view area is moved, fetches the features within that bounding box, and combines them with features previously fetched. Much of the time there will be features downloaded that already exist in the "previously fetched" dataset, so these need to be not duplicated.

**Is there a Turf operation to merge two FeatureCollections in this way?** I can't see such a thing. `union` combines two different polygons into a new one. `combine` combines Point/LineString/Polygon features into MultiPoint/MultiLineString/MultiPolygon.