The Duke of Richmond is also the Earl of March. This lesser title (Earl) is then used by the eldest son as a courtesy title. When the original Duke dies the eldest son becomes Duke of Richmond and his son, if he has one, becomes Earl of March. Read all about courtesy titles on wikipedia, which is a very useful resource despite what some insist:
http://en.wikipedia.org/wiki/Courtesy_title
Up the Republic!
I should have added that Frederick Charles Gordon-Lennox ( Earl of March) became Duke of Richmond on 7th May 1935, when his father died.
This is a good site for births, deaths and accessions of the English aristocracy:
http://www.angeltowns.com/town/peerage/