Duplicate Values in the index, primary key or relationship

Could you tell me what I need to do to get this not to give me an error?

The changes you requested to the table were not successful because they
would create duplicate values in the index, primary key, or relationship.
Change the date in the field or fields that contain the duplicate date,
remove the index, or redefine the index to permit duplicate entries and try
again.

thanks.
--
Denise B.