Error occurs when use osmosis to import data into apidb.

When i run osmosis --read-pbf planet/tianjin_china.osm.pbf --write-apidb host="localhost" database="openstreetmap" user="osm" password="buaanlp" validateSchemaVersion="no" command, some errors appeared. It said duplicate key value violates unique constraint "nodes_pkey"

I have used this command to import beijing_china.osm.pbf into the apidb, and the same errors occurred. I drop the database and create it again, then the error disappeared. But when i run the command secondly, the error still occurs, how to solve this problem? I don't want to drop the database again.