A duplicate key is a key field that is exactly the same in two or more records in the master file. The Index command's DUPKEYS parameter allows you to choose whether or not to receive a warning message if duplicate keys are found in the index file. Including duplicate keys does not cause duplicate entries in the index file, as the relative numbers for each entry are different.
With or without the warning message, the Index command always creates an index file which always includes the duplicate keys. The Index command's LISTDUPKEYS parameter can be used to list all duplicate keys. This optional listing is sent to the screen by default, but can also be sent to a printer or display file.