Prevent Data insert if the data is duplicate or already exists in sql server table in c#

HI

this my table in database

S_register

and in register.aspx page I have some TB that users can enter their data on it :

SP

ALTER procedure [dbo].[insertSreg]
@Name nvarchar(40),
@Address nvarchar(Max@Mobile nvarchar(20),@Success NVARCHAR(60) OUTPUT


as

begin
IF NOT EXISTS(SELECT id FROM S_register WHERE Mobile = @Mobile) 
begin

insert into S_register (Name,Address
Mobile)
values(@Name,@Address
@Mobile)
end
ELSE
    SET @Success = 'please select other name.'

end


 

Behind code

protected void ImageButton1_Click(object sender, ImageClickEventArgs e)
    {
       
            SqlCommand _cmd = new SqlCommand("insertSreg", _cn);
            _cmd.CommandType = CommandType.StoredProcedure;
            _cn.Open();

            _cmd.Parameters.AddWithValue("@Name", txtname.Text);
         
            _cmd.Parameters.AddWithValue("@Address", txtDname.Text);
         
   
           
            _cmd.Parameters.AddWithValue("@Mobile", Txtmob.Text);
           
            _cmd.Parameters.Add("@Success", SqlDbType.NVarChar, 60);
            _cmd.Parameters["@Success"].Direction = ParameterDirection.Output;

            _cmd.ExecuteNonQuery();
           

            Response.Redirect("success.aspx");


            _cn.Close();
            LBLERROR.Text = _cmd.Parameters["@Success"].Value.ToString();

        


    }

I want when users enter their Name in TB if in above Table was same name it don't insert any data in Table and show error that "Please Select Othername" @success parameter that I define in SP

and if they don't enter duplicate name in TB it insert data in table and go to success.aspx page

but here when users enter duplicate name it don't insert data in table  but it don't show error message (@success parameter) and go to success.aspx page?I want if they enter duplicate name it showw error and doesn't go to success.aspx page

What should i do?

Best Regards