Duplicated root when serializing

I am trying to serialize a dictionary into xml, but the structure of the xml is different to the one I would like. For some reason instead of having one root and multiple child within that root I get a new root for each key and value in the dictionary. Also the message box is being shown for each KeyValuePair within the dictionary, I would like the message box to be shown once when the dictionaries were exported. I am using Microsoft Visual Studio 2010 with C# and all values within the dictionary collected from text boxes on a windows form.

This is how the current xml layout looks like:

<?xml version="1.0" encoding="utf-8"?>
<Coordinate xmlns:xsi="http://ift.tt/ra1lAU" xmlns:xsd="http://ift.tt/tphNwY">
   <Date_Time>10/02/2015 11/17/00</Date_Time>
   <BeaconID>1</BeaconID>
   <X_Coordinate>2</X_Coordinate>
   <Y_Coordinate>3</Y_Coordinate>
</Coordinate><?xml version="1.0" encoding="utf-8"?>
<Coordinate xmlns:xsi="http://ift.tt/ra1lAU" xmlns:xsd="http://ift.tt/tphNwY">
   <Date_Time>10/02/2015 11/17/01</Date_Time>
   <BeaconID>2</BeaconID>
   <X_Coordinate>3</X_Coordinate>
   <Y_Coordinate>4</Y_Coordinate>
</Coordinate>

And this is how I would like it to look:

<?xml version="1.0" encoding="utf-8"?>
<Coordinates xmlns:xsi="http://ift.tt/ra1lAU" xmlns:xsd="http://ift.tt/tphNwY">
    <Coordinate>
        <Date_Time>10/02/2015 11/17/00</Date_Time>
        <BeaconID>1</BeaconID>
        <X_Coordinate>2</X_Coordinate>
        <Y_Coordinate>3</Y_Coordinate>
    </Coordinate>
    <Coordinate>
        <Date_Time>10/02/2015 11/17/01</Date_Time>
        <BeaconID>2</BeaconID>
        <X_Coordinate>3</X_Coordinate>
        <Y_Coordinate>4</Y_Coordinate>
     </Coordinate>
</Coordinates>

This is all of my xml related code:

public void btn_Submit_Click(object sender, EventArgs e)
    {
        foreach (KeyValuePair<string, Tuple<int, int>> entry in d)
        {
            Coordinate v = new Coordinate();

            v.Date_Time = DateTime.Now.ToString("dd/MM/yyyy hh/mm/ss");
            v.BeaconID = entry.Key;
            v.X_Coordinate = entry.Value.Item1.ToString();
            v.Y_Coordinate = entry.Value.Item2.ToString();

            SaveValues(v);
        }
    }

    public class Coordinate//Root element
    {
        public string Date_Time { get; set; }
        public string BeaconID { get; set; }
        public string X_Coordinate { get; set; }
        public string Y_Coordinate { get; set; }
    }

    public void SaveValues(Coordinate v)
    {
        XmlSerializer serializer = new XmlSerializer(typeof(Coordinate));
        using (TextWriter textWriter = new StreamWriter(@"F:\Vista\Exporting into XML\Test1\Coordinates output.xml", true))
        {
            serializer.Serialize(textWriter, v);
        }
        MessageBox.Show("Coordinates were exported successfully", "Message");//Let the user know the export was succesfull            
    }

Can you tell me what I am doing wrong. If I am missing anything or I just wasn't clear enough just let me know it and I will help with it. I am new to XML and to serialization so any help would be appreciated.