how to prevent bug/error when saving data with the same primary key on visual basic 2012?

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VB

SQL-Server-2012

i'm using visual studio 2012 and microsoft SQL server 2012 to make a save function.

my coding is like this:

form:

Private Sub btnSave_Click(sender As Object, e As EventArgs) Handles btnSave.Click
        If Len(Trim(txt_nis.Text)) = 0 Or Len(Trim(txt_nisn.Text)) = 0 Or Len(Trim(txt_namasiswa.Text)) = 0 Or Len(Trim(cmb_kelaminsiswa.Text)) = 0 Or Len(Trim(txt_kotalahir.Text)) = 0 Or Len(Trim(DTP_siswa.Value)) = 0 Or Len(Trim(cmb_agamasiswa.Text)) = 0 Or Len(Trim(txt_beratsiswa.Text)) = 0 Or Len(Trim(txt_tinggisiswa.Text)) = 0 Then
            MsgBox("Data Belum Lengkap")
            Exit Sub
        End If
        ClassSiswa.Nis = txt_nis.Text
        ClassSiswa.Nisn = txt_nisn.Text
        ClassSiswa.Nama_Siswa = txt_namasiswa.Text
        ClassSiswa.Jenis_Kelamin = cmb_kelaminsiswa.Text
        ClassSiswa.Kota_Lahir = txt_kotalahir.Text
        ClassSiswa.Tanggal_Lahir = DTP_siswa.Value
        ClassSiswa.Agama = cmb_agamasiswa.Text
        ClassSiswa.Berat_Badan = txt_beratsiswa.Text
        ClassSiswa.Tinggi_Badan = txt_tinggisiswa.Text
        ClassSiswa.RekamData(ClassSiswa.opencon)
        MsgBox("Data siswa Berhasil Disimpan")
        datagridview()
    End Sub

CLASS:

Public Shared Sub RekamData(ByVal _Cn As SqlClient.SqlConnection)
        Dim sql As SqlClient.SqlCommand
        sql = New SqlClient.SqlCommand
        sql.Connection = _Cn
        sql.CommandType = CommandType.Text
        sql.CommandText = "Insert into siswa values('" & Nis & "','" & Nisn & "','" & Nama_Siswa & "','" & Jenis_Kelamin & "','" & Kota_Lahir & "','" & Tanggal_Lahir & "','" & Agama & "','" & Berat_Badan & "','" & Tinggi_Badan & "')"
        sql.ExecuteNonQuery()
    End Sub

SQL Query:

Create Table siswa 
(
Nis varchar (40) primary key,
Nisn varchar (40),
Nama_Siswa varchar(40),
Jenis_Kelamin varchar (10),
Kota_Lahir varchar (10),
Tanggal_Lahir date,
Agama varchar (10),
Berat_Badan varchar (10),
Tinggi_Badan varchar (10)
)

Nis is primary key.

in the windows form datagridview there is a data that i already saved with Nis : 123, if i saved another data with the same Nis : 123 the program will stop and give me a error because there is already Nis with the value 123.

what i wanted to do is:

instead of show me the error, i want to make a coding that show me a messagebox "data already saved" so that the program won't stopped beacuse of error.

how do i make a coding to do that?