The Collaborative International Dictionary of English v.0.48:

Dukedom \Duke"dom\, n.
   1. The territory of a duke.
      [1913 Webster]

   2. The title or dignity of a duke. --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

dukedom
    n 1: the dignity or rank or position of a duke
    2: the domain controlled by a duke or duchess [syn: duchy,
       dukedom]

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "dukedom":
   ally, archduchy, archdukedom, baronetcy, barony, body politic,
   buffer state, captive nation, chieftaincy, chieftainry, city-state,
   colony, commonweal, commonwealth, country, county, domain,
   dominion, duchy, earldom, empery, empire, free city, grand duchy,
   kingdom, kingship, knight-errantship, knighthood, ladyship, land,
   lordship, mandant, mandate, mandated territory, mandatee,
   mandatory, marquisate, nation, nationality, pashadom, polis,
   polity, possession, power, princedom, princeship, principality,
   principate, protectorate, province, puppet government,
   puppet regime, queenship, realm, republic, satellite, seignioralty,
   seigniory, seneschalty, settlement, sovereign nation, state,
   sultanate, superpower, territory, toparchia, toparchy, viscountcy,
   viscountship, viscounty