There is a file with name “_rm_dup_<dup_db>.dat” under $ORACLE_HOME/dbs of clone database. This file is from executing “duplicate target database for standby from active database”.
“_rm_dup_<dup_db>.dat” stores the names of datafilecopy already created by duplicate. Inside this file, rman can find the name of the datafiles already copied to auxiliary host. if, for some reasons, the duplicate failed, the second duplicate will check this file. If the datafilecopy has been created on auxilary host, and the vital information of the datafile (file number, database id, creation scn, database name) and its checkpoint scn is behind the until scn, then the datafilecopy can be used by this new duplicate and restore/copy is not necessary.
So it is for “restore optimization” purpose when “duplicate target database for standby from active database”.