Error in Geolocator in Xamarin.Forms.Map integration

I have a tabbed app and in one of the tab I have map integrated in it and everytime switch the tabs I get an error saying "The geolocator is already listening".

protected async override void OnAppearing()
    {
        base.OnAppearing();

        var locator = CrossGeolocator.Current;

        locator.PositionChanged += Locator_PositionChanged;

        await locator.StartListeningAsync(TimeSpan.FromSeconds(0),50,true);

        var position = await locator.GetPositionAsync();
        var center = new Xamarin.Forms.Maps.Position(position.Latitude, position.Latitude);
        var span = new Xamarin.Forms.Maps.MapSpan(center, 2, 2);

        locationsMap.MoveToRegion(MapSpan.FromCenterAndRadius(new Position(position.Latitude, position.Longitude),
                                         Distance.FromMiles(1)));
    }


    void Locator_PositionChanged(object sender, Plugin.Geolocator.Abstractions.PositionEventArgs e)
    {
        var center = new Xamarin.Forms.Maps.Position(e.Position.Latitude, e.Position.Latitude);
        var span = new Xamarin.Forms.Maps.MapSpan(center, 2, 2);

        locationsMap.MoveToRegion(MapSpan.FromCenterAndRadius(new Position(position.Latitude, position.Longitude),
                                         Distance.FromMiles(1)));
    }