21. Double overprints or surcharges: These are quite rare. One is the $2-on-$l Shanghai surcharge (Scott Kl6a). The other is a Minneapolis, Minn., precancel on the 1954 8¢ Liberty issue. The first catalogs several thousand dollars mint or used, while the second is more in the $100 curiosity range.
Type 21: The Shangai overprint (Scott K16a) is found with a doubled surcharge.