a = {{1.1, 2.2}, {1.1, 2.2}, {x, b}, {c, d}, {x, b}, {1, 2}}
# /. (Rule[First@#, Append @@ #] & /@ Tally[#]) &[a]
(*
{{1.1, 2.2, 2}, {1.1, 2.2, 2}, {x, b, 2}, {c, d, 1}, {x, b, 2}, {1, 2,1}}
*)
For huge lists:
# /. Dispatch[Rule[First@#, Append @@ #] & /@ Tally[#]] &[a];
Will speed things up nicely.