Merging two rows with rounding their first elemnts

lst = { {-0.410927, {{11, 16}, {12, 16}, {15, 5}, {15, 6}}},
   {-0.41, {{11, 16}, {12, 16}, {15, 5}, {15, 6}}},
    {-0.45123, {{11, 16}, {12, 16}, {15, 5}, {15, 6}}},
 {-0.410927, {{9, 14}, {10, 14}, {13, 7}, {13, 8}}}};

Values@GroupBy[lst,  Round[First@#, .001] &, {#[[1, 1]], Join @@ #[[All, 2]]} &]

{{-0.410927, {{11, 16}, {12, 16}, {15, 5}, {15, 6}, {9, 14}, {10,     14}, {13, 7}, {13, 8}}},
{-0.41, {{11, 16}, {12, 16}, {15,     5}, {15, 6}}},
{-0.45123, {{11, 16}, {12, 16}, {15, 5}, {15, 6}}}}

Values@GroupBy[lst,  Round[First@#, .01] &, {#[[1, 1]], Join @@ #[[All, 2]]} &]

{{-0.410927, {{11, 16}, {12, 16}, {15, 5}, {15, 6}, {11, 16}, {12, 16}, {15, 5}, {15, 6}, {9, 14}, {10, 14}, {13, 7}, {13, 8}}},
{-0.45123, {{11, 16}, {12, 16}, {15, 5}, {15, 6}}}}

lst={{-0.410927,{{11,16},{12,16},{15,5},{15,6}}},{-0.41,{{11,16},{12,16},{15,5},{15,6}}},{-0.45123,{{11,16},{12,16},{15,5},{15,6}}},{-0.410927,{{9,14},{10,14},{13,7},{13,8}}}};

{#[[1,1]],Flatten[#[[All,2]],1]}&/@Values[GroupBy[lst,SetPrecision[#[[1]],3]&]]   

{{-0.410927, {{11, 16}, {12, 16}, {15, 5}, {15, 6}, {9, 14}, {10, 14}, {13, 7}, {13, 8}}}, {-0.41, {{11, 16}, {12, 16}, {15, 5}, {15, 6}}}, {-0.45123, {{11, 16}, {12, 16}, {15, 5}, {15, 6}}}}