Method as Event Handler

Question

can anyone help with my problem ? I have 2 classes.

public partial class StationTabItem : UserControl

{
SessionServiceClass.Instance.getHistoricalStationData(Convert.ToUInt32(station.stationNumber), setHistoricalStationData);    
 public void setHistoricalStationData(object sender, readStationDataHistoryCompletedEventArgs e)
    {

        if (!e.Cancelled && (e.Error == null))
        {
            historicalStationData = new List<StationData>();
            historicalStationData = e.Result.ToList();
            fillHistoricalData(historicalStationData);

            InitializeComponent();
            ComboBox_Left.SelectedIndex = leftIndex;
            ComboBox_Right.SelectedIndex = rightIndex;
            TextBlock_StationName.Text = stationName;
            TextBox_DetailsInfo.Text = evidUdajeStanice;
            fillStationData(station);
            updateDataGrids(localDynamicData_weatherData.variables, localDynamicData_alignedSurfaceData.variables, localDynamicData_oppositeSurfaceData.variables);

        }
    }

and second class

public class SessionServiceClass
{
public void getHistoricalStationData(uint stationID, EventHandler<readStationDataHistoryCompletedEventArgs> setHistoricalStationData)
    {
        rwisClient.readStationDataHistoryAsync(stationID, System.DateTime.Today.AddHours(System.DateTime.Now.Hour).AddMinutes(System.DateTime.Now.Minute), -86400);

        rwisClient.readStationDataHistoryCompleted -= setHistoricalStationData;
        rwisClient.readStationDataHistoryCompleted += setHistoricalStationData;

    }
}

The problem is, if I create more instances of StationTabItem, allways is called every method setHistricalStationData in every instance but with result ,,e.result,, of last istances. It means my variable historicalStationData is overwrite as is last value of it. Thanks in advance for any ideas.