how to insert data in tables in vb.net but avoid duplication

Please i would ask you to post in your efforts first and then ask for assistance as connectivity is the basic along with DML which is thought in colleges or class as part of .NET....

Now coming to your query

use a primary key constraint with your table for the value which you want to keep unique
now coming to the connectivity part hers the code

Imports System.Data.SqlClient 'use this namespace

Public Class Form1

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        ' it will first check whether the textbox contains data or not
        If TextBox1.TextLength = 0 Then
            'if textbox does not contain data then prompt the user to enter data
            'with the help of messagebox
            MessageBox.Show("value missing")
        Else
            'creating the object of sqlconnection class and setting its
            'connectionstring property
            Dim con As SqlConnection = New SqlConnection("Data Source=.;Initial Catalog=master;Integrated Security=True")
            'open the connection
            con.Open()
            'set the command by creating the object of sqlcommand class
            Dim cmd As SqlCommand = New SqlCommand("insert into mytable values('" + TextBox1.Text + "')", con)
            'execute the query
            Dim x As Integer = cmd.ExecuteNonQuery()
            If (x <> 0) Then
                'once the data is added display this messagebox to user
                MessageBox.Show("data added")
            End If
            'close the connection in the end
            con.Close()
        End If
    End Sub
End Class

Do rate my answer once you find it useful
Thanks & Regards
Radix :rose: