My Session Variable doesn't store the correct value

Question

I have a session variable stored from when a User logs into the web app. Then the logged in user can store information. When this information stores the Session Variable(UserId) doubles itself

 

for example if the user ID is 1 it saves as 11 or if it should be 4 it saves as 44.

  

Any ideas why this might be the case.

 

I have the session variable saved into a label on the form when the page loads:

 

//show label with corresponding Name used in log in

UserID.Text += Session["UserId"].ToString();

 

This is my code behind the save button:

protected void AddBooking_Click(object sender, EventArgs e)
{
//Creating a connection to my database using the connection string
string cs = ConfigurationManager.ConnectionStrings["BookingDb"].ConnectionString;
SqlConnection con = new SqlConnection(cs);
//preparing a query which will insert the data entered in the textboxes to the database
SqlCommand cmd = new SqlCommand("[dbo].[spAddBooking]", con);
cmd.CommandType = CommandType.StoredProcedure;
try
{
cmd.Parameters.AddWithValue("@UserId", UserID.Text.Trim());
cmd.Parameters.AddWithValue("@MemberType", ddlMemberType.SelectedValue);
cmd.Parameters.AddWithValue("@PitchSection", ddlSyntheticPitch.SelectedValue);
cmd.Parameters.AddWithValue("@Date", txtDate.Text.Trim());
cmd.Parameters.AddWithValue("@StartTime", ddlStartTime.SelectedValue);
cmd.Parameters.AddWithValue("@EndTime", ddlEndTime.SelectedValue);
cmd.Parameters.AddWithValue("@Description", txtDescription.InnerText.Trim());
con.Open();
cmd.ExecuteNonQuery();
}
catch (Exception ex)
{
Response.Write(ex);
}
finally
{
con.Close();
Response.Redirect("BookingForm.aspx");
}