l = Table[RandomInteger[{1, 10^6}], {1000}];
ls = Sort[l];
index = -1; Do[If[ls[[i]] == ls[[i + 1]], index = i], {i, 1, Length[ls] - 1}];
pos = Position[Partition[l, 20], ls[[index]]];

Grid[Partition[l, 20], Frame -> All, FrameStyle -> Lighter[Gray], Background -> {None, None, If[index != -1, {pos[[1]] -> Lighter[Orange],
      pos[[2]] -> Lighter[Orange]}, None]}] //
 Style[#, FontSize -> 9] &