Ordinarily, makemap will complain if two entries have identical keys and refuse to insert the duplicate. But if it is desirable to allow the same key to appear multiple times in the database, you can use the -d switch to suppress the warnings and allow duplicates to be inserted. But be aware that this switch is allowed only for the btree and hash forms of the db(3) library. Use of this switch with any other form of database will produce the following error:

makemap: Type dbtype does not support -d (allow dups)

See the -r switch for a way to cause duplicates to replace originals.