Luis
I read a little more about the Duke avocado in these articles.
Look at the second part to your link above.
They referred to a blog that a number of people made comments.

One comment stood out that I will quote here:

Bill Bird2 years ago

A Duke seed will not produce a true Duke Avocado. The only way to get a true Duke is to graft cuttings from a true Duke tree and graft it to the root stock.

You may have to reconsider your options about growing a seed. Also this person had a difficult time grafting the Duke tree stock. His three tries at it didn't take and he lost the root stock and all.