Richard, the rules and criteria for awarding the Medal were revised and awarded medals were reviewed in 1897. Three medals, awarded to Daniel Sickles, Henry Tremain, and Daniel Butterworth (the so-called "Sickles' Circle) forced the review of the process in 1897 .Because these high-ranking individuals had gained medals in questionable ways, and because of other abuses of the Civil War era medal nominations, the process of applying for a Medal of Honor was revised.
The Purge of 1917
Perhaps the single most famous event associated with the Medal of Honor is the Purge of 1917. Originally convened in 1916 by Nelson Miles, himself a MOH awardee, the commission reviewed each of the Army medals awarded. Their report, presented in February, 1917, revoked the medals presented to 911 people including 864 medals awarded to the 27th Maine for re-enlisting and President Lincoln's funeral guard. Six medals awarded to civilians were revoked as well. Included in this group were Mary Walker, the only female awardee, and Buffalo Bill Cody, a scout and technically not a soldier during the Indian Wars.
The precedent was set for group recognition in 2001 by Goerge W. Bush.
During the Second World War a group of Navajo served the front lines relaying coded messages back to rear echelon support groups. These "Code Talkers" were routinely in the line of fire and performed their assigned task with merit -- duty that frequently brought them into jeopardy. It was felt that the heroic action of the "Code Talkers" had been overlooked. Congress approved a special gold medal to be awarded these Navajo Indians who served on the battlefield during World War II. These medals were presented to the living recipients by President George W. Bush on July 26, 2001.
The above info can be found at the Medal of Honor Museum in Chattanooga, TN.
http://ngeorgia.com/tenn/mohm.html