Alternatively, you have tried to access an old format indexed file, created perhaps using CIS COBOL.
Resolution:
If your index has no room for further entries you should either reorganize your file or convert it to IDXFORMAT"4" using Rebuild, to allow a greater number of duplicate keys.
If you have tried to access an old format indexed file, you can run the Rebuild utility to check the consistency of this indexed file, and to construct a new indexed file if the old one was found to be corrupt.