Script to merge layers that have the same name into sublayers

New layers with sublayers are created on top of other layers.

// Script to merge layers that have the same name into sublayers
// https://community.adobe.com/t5/illustrator-discussions/script-to-merge-layers-that-have-the-same-name-into-sublayers/td-p/14968548

function main() {
  if (!documents.length) return;
  var doc = app.activeDocument;
  var layerGroups = collectSameLayers(doc);
  moveLayersToNew(doc, layerGroups);
}

// Collect layers with the same name
function collectSameLayers(doc) {
  var layerGroups = [];

  for (var i = 0; i < doc.layers.length; i++) {
    var currLayer = doc.layers[i];
    var currName = currLayer.name;
    var isFound = false;

    for (var j = 0; j < layerGroups.length; j++) {
      if (layerGroups[j].name === currName) {
        layerGroups[j].layers.push(currLayer);
        isFound = true;
        break;
      }
    }

    if (!isFound) {
      layerGroups.push({
        name: currName,
        layers: [currLayer]
      });
    }
  }

  return layerGroups;
}

// Create new layer and move inside original layers
function moveLayersToNew(doc, layerGroups) {
  for (var i = layerGroups.length - 1; i >= 0; i--) {
    var currGroup = layerGroups[i];

    // Only process if there are multiple layers with the same name
    if (currGroup.layers.length > 1) {
      var newLayer = doc.layers.add();
      newLayer.name = currGroup.name;

      for (var j = currGroup.layers.length - 1; j >= 0; j--) {
        currGroup.layers[j].move(newLayer, ElementPlacement.INSIDE);
      }
    }
  }
}

try {
  main();
} catch (e) {}