I guess just a never fixed typo, in the first explanation of the mentioned lesson the text says The Najdorf Defense, 1.e4 c5 2.Nf3 d6 3.d4 cxd 4.Nxd4 Nf6 5.Nc3 d6 has been a popular choice for Black for over half a century.
Obviously the fifth move of black is a6 (the trademark move of the Najdorf) and not the already played d6.
I guess just a never fixed typo, in the first explanation of the mentioned lesson the text says The Najdorf Defense, 1.e4 c5 2.Nf3 d6 3.d4 cxd 4.Nxd4 Nf6 5.Nc3 d6 has been a popular choice for Black for over half a century.
Obviously the fifth move of black is a6 (the trademark move of the Najdorf) and not the already played d6.