The message indicates that your table has an index that is either the
Primary Key or has its Unique Values property set to Yes. As a result,
duplicate values are not permitted in indexed field(s).
For example, say you have a table of clients. In table design view, you
selected the Surname field, and in the lower pane, set the Indexed property
to:
Yes (No Duplicates.)
You entered some records, including Mr Fred Jones.
Later, when you try to enter Mrs Martha Jones, you will get the error.
