Code:

            MapElement selectedElement = e.Element;
            selectedElement.Layer.DerenderElement(selectedElement);
            SqlShapeReader read = selectedElement.Layer.Reader as SqlShapeReader;
            IEnumerable<Dictionary<string, string>> buffer = read.DataSource as IEnumerable<Dictionary<string, string>>;
            Dictionary<string, string> buf = buffer.Where(b => b["IDENT"] == selectedElement.Name).Single();

            this.IsEnabled = false;
            ChangeObr changeWin = new ChangeObr(buf);
            changeWin.btnSave.Click += (s, o) =>
            { 
                string layerName = "layer" + buf["comp_obr"];
                SqlShapeReader newReader = xamMap.Layers[layerName].Reader as SqlShapeReader;
                List<Dictionary<string, string>> source = new List<Dictionary<string, string>>();               
                source = newReader.DataSource as List<Dictionary<string, string>>;
                source.Add(buf);
                newReader.DataSource = source;
                newReader.ImportAsync(newReader.Layer);
                xamMap.Layers[layerName].RenderElement(selectedElement);
                if(changeWin != null) changeWin.Close();
                this.IsEnabled = true;
                xamMap.Layers[layerName].RenderWindow();
            };
            changeWin.btnCancel.Click+= (s, o) =>
            {
                this.IsEnabled = true;
                selectedElement.Layer.RenderElement(selectedElement);
            };
            changeWin.ShowDialog();

Explanation: When I click on a map element I get it's data source (it is Dictionary<string,string> with 4 pairs<Key,Value> - one of them is the sql geometry type) then I send this item to the new window where I change the comp_obr of the Company (which determines the layer this element belongs to) and derender it from the layer. Then if I don't save the new comp_obr I just render this element again. Otherwise if I save the new comp_obr I get the edited item and add it to the source of the new layer and render the layer. But in the way I have written I duplicate the selected element. I understood that by the number of elements of the new and the old layer of the element. The new has increased by 1 element, but the hasn't decreased by 1. My aim is to change the item's comp_obr value and visually to show this change by moving this element to the new layer. Thanks in advance!