arr = Table[Random[], {5000}, {5}, {12}, {12}];

ans = Map[ArrayFlatten,
    Map[{{#, #}, {#, #}} &, arr, {4}], {2}]; // AbsoluteTiming

(* {2.6501516, Null} *)

ans // Dimensions

(* {5000, 5, 24, 24} *)

ans[[3, 4, 1 ;; 5, 1 ;; 5]] // MatrixForm