The Collaborative International Dictionary of English v.0.48:

Sameness \Same"ness\, n.
   1. The state of being the same; identity; absence of
      difference; near resemblance; correspondence; similarity;
      as, a sameness of person, of manner, of sound, of
      appearance, and the like. "A sameness of the terms." --Bp.
      Horsley.
      [1913 Webster]

   2. Hence, want of variety; tedious monotony.
      [1913 Webster]

   Syn: Identity; identicalness; oneness.
        [1913 Webster]

WordNet (r) 3.0 (2006):

sameness
    n 1: the quality of being alike; "sameness of purpose kept them
         together" [ant: difference]
    2: the quality of wearisome constancy, routine, and lack of
       variety; "he had never grown accustomed to the monotony of
       his work"; "he was sick of the humdrum of his fellow
       prisoners"; "he hated the sameness of the food the college
       served" [syn: monotony, humdrum, sameness]

Moby Thesaurus II by Grady Ward, 1.0:

85 Moby Thesaurus words for "sameness":
   accordance, agreement, alikeness, alliance, analogy, aping,
   approach, approximation, assimilation, broken record,
   clockwork regularity, closeness, coequality, coincidence,
   community, comparability, comparison, conformity, congruence,
   constancy, copying, correspondence, daily round, dingdong, drone,
   equality, equivalence, equivalency, even pace, even tenor,
   homogeneity, homoousia, humdrum, identity, imitation,
   indistinguishability, invariability, irk, irksomeness, likeness,
   likening, metaphor, mimicking, monologue, monotone, monotonousness,
   monotony, nearness, no difference, oneness, orderliness, par,
   parallelism, parity, regularity, resemblance, sameliness,
   self-identity, selfhood, selfness, selfsameness, semblance,
   similarity, simile, similitude, simulation, singsong, smoothness,
   synonymity, synonymousness, synonymy, tedium, the beaten track,
   the daily round, the round, the squirrel cage, the treadmill,
   the weary round, treadmill, undeviation, undifferentiation,
   uniformity, unity, unvariation, wearisome sameness