Duplicate key error on Append Query

[Lisa Wilder]

I have a database that I use to append data to a table
that is linked to a table in another program using ODBC.
This has worked fine for the last year or so, now I'm
suddenly getting a error that says:

ODBC--insert on a linked table failed. [ODBC Driver][DLL]
Illegal duplicate key. (#-1605)

Does anyone know what caused this, and how I can fix it?

Thanks,
Lisa Wilder