In the upcoming Schulman BV auction 353 (June 24, 2017), a unique Kampen ducat 1654 over 1653 will be available for bidding (Lot 258). Estimated price is 360 EUR.
Following description in the auction catalogue, this overdate is not recorded in any archive, thus it seems to be the only copy of this date known to exist. In addition, in general overdated ducats from the Campen mint are extremely rare.
You may bid this coin here.