add putDuplicateTile function

lib/mbtiles.js

@@ -562,6 +562,31 @@ MBTiles.prototype.putTile = function(z, x, y, data, callback) {
     else return callback(null);
 };
 
+// Inserts a duplicate tile into the MBTiles store. Scheme is XYZ.
+//
+// - @param {Number} z tile z coordinate
+// - @param {Number} x tile x coordinate
+// - @param {Number} y tile y coordinate
+// - @param {Number} duplicate tile ID
+// - @param {Function(err)} callback
+MBTiles.prototype.putDuplicateTile = function(z, x, y, id, callback) {
+    if (typeof callback !== 'function') throw new Error('Callback needed');
+    if (!this.open) return callback(new Error('MBTiles not yet loaded'));
+    if (!this._isWritable) return callback(new Error('MBTiles not in write mode'));
+
+    // Flip Y coordinate because MBTiles files are TMS.
+    y = (1 << z) - 1 - y;
+
+    // This corresponds to the map table.
+    var coords = hash(z, x, y);
+    if (!this._mapCache[coords]) this._mapCache[coords] = { z: z, x: x, y: y };
+    this._mapCache[coords].tile_id = id;
+
+    // Only commit when we can insert at least batchSize rows.
+    if (++this._pending >= this._batchSize) return this._commit(callback);
+    else return callback(null);
+};
+
 // Inserts a grid into the MBTiles store. Scheme is XYZ.
 //
 // - @param {Number} z grid z coordinate