Select lists where last elements are identical

Preserving the order:

With[{c = CountsBy[t1, Last]}, Select[t1, c[Last@#] > 1 &]]

If order is not important

t1 // GroupBy[Last] // Select[Length[#] > 1 &] // Values

(* {{{5, 5, 50}, {7, 1, 50}}, {{7, 4, 65}, {8, 1, 65}}} *)

GroupBy[Counts[t1[[All, -1]]]@#[[-1]] > 1 &][t1]@True

  {{5, 5, 50}, {7, 4, 65}, {8, 1, 65}, {7, 1, 50}}