want to prevent duplication in records using C# with the help of code

You cant use the SqlDataReader for a second request while the first request is not jet finished (Close() command). Better have a second SqlDataReader object for the second query.

You use opened datareader and execute command at the same time.
In this case you could replace following block of code:

SqlCommand cmd = new SqlCommand("select * from account",con);
SqlDataReader dr = cmd.ExecuteReader();
if (dr.Read().Equals(textBox1.Text))

With something like this:

var cmd = new SqlCommand("SELECT COUNT (*) FROM [account] WHERE fieldname = @fieldname",con);
cmd.Parameters.AddWithValue("@fieldname", textBox1.Text);
var isExists = Convert.ToInt32(cmd.ExecuteScalar()) > 0;
if (isExists)

This will solve problem, just replace "fieldname" with name of field in db table to which you compare textBox1.Text.

Some small recommendations:
1. Do not use non-prepared strings in your sql queries (problem of sql-injection). Use parameters instead;
2. Store connection string in application configuration file in <connectionstrings> section;
3. Provide more attention to disposable resources. In case of any problem in try block db connection will stay open;
4. Use string.Format or StringBuilder for string concatenation;
If it's just test project or example then never mind :)

What I usualy do and am not saying its the best, but I read stuff from the database and compare the stuff that i have with wat i want to insert. E.g i will check if the password does not exist in my database

 DataTable dTable = new DataTable();
dTable = systemBusinessLayer.GetAllEmployeesInfo();

foreach (DataRow dRow in dTable.Rows)//Go through each row inside datatable
            {

                
    if (textBox1.Text== dRow["Password"].ToString())
                {
    MessageBox.Show("The password already exist");
                }
                 else
                    {
                       MessageBox.Show("The password does not exist");
                    }
            }

Now systemBusinessLayer is my Class where you can find the method getAuthentication()

public DataTable getAuthentication()
       {
           using (SqlConnection con = new SqlConnection(ConnString))
           {
               SqlCommand cmd = new SqlCommand("procGetLoginDetails", con);
               cmd.CommandType = CommandType.StoredProcedure;


               DataTable dTable = new DataTable();
               SqlDataAdapter adapter = new SqlDataAdapter(cmd);
               adapter.SelectCommand = cmd;
               adapter.Fill(dTable);

               return dTable;


           }


       }

then

procGetLoginDetails

is my storedprocedure that has the sql query.

Hope it helps...