Osmium tool: merge pbf

I had some problems importing with osm2pgsql hungary-latest.osm.pbf with the --append command, in a existent db: there was a duplicate key error on planet_osm_relations.
So, I read this topic: https://forum.openstreetmap.org/viewtopic.php?id=58891 and I merged with osmium-tool:

  • hungary-latest.osm.pbf
  • italy-latest.osm.pbf
  • gcc-states-latest.osm.pbf
  • rwanda-latest.osm.pbf
    in one file.

I thought this operation could resolve the duplicate key error, but… during the import with osm2pgsql, again, the error:

DETAIL:  Key (id)=(214885) already exists. 
CONTEXT:  COPY planet_osm_rels, line 12093

So… how can i resolve this? What did I do wrong?