[belajar-access] Membuat msgbox jika memasukan duplikat data

Dear pakar access, mohon bantuannya..bagaimana membuat pesan box jika kita memasukan data yang duplikat, misal :
Isi tabel
IDPelanggan Nama Alamat
12B Joko Jl. Singa
13B Ali Jl. Bangau
dll

Jika kita memasukan data di form lalu kita memasukan lagi data IDPelanggan 12B, lalu saat mau disimpan di command button, muncul pesan the changes you requested to the table were not succesful bla bla bla....
gimana caranya mengganti pesan error tadi menjadi messagebox misal :
"Data yang anda masukan sudah ada, mohon di input IDPelanggan yang lain"

Terima kasih

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