Message Oracle ORA-38773 - cannot add data file string - file already part of database

ERRORcannot add data file string - file already part of database
CAUSEThe recovery step of FLASHBACK database encountered the creation of a datafile and could not create the file because the file name is already in use in the database. The control file file entry for the file is UNNAMEDnnnnn.
ACTIONUse the ALTER DATABASE CREATE DATAFILE statement to create the file with a different name.