Forms Data Controls :: GridView's RowDataBound Event Fired Twice?

I have a very simple asp.net page, just some buttons and one GridView on it. The logic is a user click the QUery button,then the button click event was trigured to populate the gridview with data queried from database via LINQ. for a user friendly interface, in RowDataBound event, I visit every row, change a boolean value into checkbox represention. But it seem that the RowDataBound run twice, because the checkboxs were inserted twice for every row in one cell. so why? the source code snippet as follow:

protected void btnSelect_Click(object sender, EventArgs e)
{
RetiredEmployeeDataContext db = new RetiredEmployeeDataContext();
GridView1.DataSource = db.ENTUserAccountSelectAll();//userAccounts;
GridView1.DataBind();
}
protected void GridView1_OnRowDataBound(object sender, GridViewRowEventArgs e)
{
if (e.Row.RowType == DataControlRowType.DataRow)
{
CheckBox chkActive = new CheckBox();
chkActive.Checked = ((ENTUserAccount)e.Row.DataItem).IsActive;
chkActive.Enabled = false;
e.Row.Cells[5].Controls.Add(chkActive);
}
}