Search For Duplicate Entery In Database

Hi I have created a simple application base on the video based on the 'SQL Server 2005 Express for Beginners' video No. 8.
It is working great but I would like to add another feature; if an emial address (in my case userName)  is entered that already exists in the database I would like to display an message say you have already subscrided.
How do I search the database for a duplicate email address? Please keep it simple I am still finding my feet with aspx and c#.
Cheers 
  protected void Wizard1_FinishButtonClick(object sender, WizardNavigationEventArgs e)
{
SqlDataSource profilesDataSource = new SqlDataSource();

profilesDataSource.ConnectionString = ConfigurationManager.ConnectionStrings["profilesConnectionString1"].ToString();

profilesDataSource.InsertCommandType = SqlDataSourceCommandType.Text;

profilesDataSource.InsertCommand = "INSERT INTO staffprofiles (userName, fullName, nickName, favouriteSaying, memorableMoment, song, farewell) VALUES (@userName, @fullName, @nickName, @favouriteSaying, @memorableMoment, @song, @farewell)";

profilesDataSource.InsertParameters.Add("userName", userNameTextBox.Text);
profilesDataSource.InsertParameters.Add("fullName", fullNameTextBox.Text);
profilesDataSource.InsertParameters.Add("nickName", nickNameTextBox.Text);
profilesDataSource.InsertParameters.Add("favouriteSaying", favSayingTextBox.Text);
profilesDataSource.InsertParameters.Add("memorableMoment", memMomentTextBox.Text);
profilesDataSource.InsertParameters.Add("song", songTextBox.Text);
profilesDataSource.InsertParameters.Add("farewell", farewellTextBox.Text);

int rowsAffected = 0;

try
{
rowsAffected = profilesDataSource.Insert();
}
catch (Exception ex)
{
// Report a problem
Server.Transfer("submit_problem.htm");
}
finally
{
profilesDataSource = null;
}

if (rowsAffected != 1)
{
// Report a problem
Server.Transfer("submit_problem.htm");
}
else
{
// Everything was OK
Server.Transfer("submit_success.aspx");
}
}