I've formatted my mac, and I've reinstalled Snow Leopard from installation CD. Successively I've accessed to the mac store to download Lion again and install it on top of the fresh Snow Leopard installation, and suprise: the mac store asks me to buy it again.
I'm using the same user profile (Apple ID).
Best Answer
I can't delete this question anymore, so I provide the solution: I had 2 different accounts.
The problem is that I've used my email for one account, instead of using my nickname, and this generated the second account. I think they should recognize emails, and warn you, you already have an account with this email, to avoid issues like mine.