How to merge Oracle data
Just add a WHERE clause to your select that eliminates the duplicates:

INSERT /*+ APPEND */ INTO db1.myTable (
   SELECT * FROM db2.myTable b WHERE NOT EXISTS 
      (SELECT 1 FROM db1.myTable a WHERE a.ID = b.ID)
   );


If you don't have a field like ID that is unique across the two tables, then you will need to compare more fields, perhaps all of them.