Yes we do know that, based on collectors releases and based on the percentage of collectors in the fan communities. Look around, how many fans do you know? How many of them buy each album multiple times? And how many of those have more copies of Invincible than any other album they got multiple copies? If you don't know fans personally (collector or not), I think one of the biggest fan boards is a great sample group. Go ahead and ask. Because if you make such a statement on a single album you really need to back this up with facts, otherwise anyone of us can make weird claims about Michael and then say "we don't know for sure so you can't say I'm wrong."
Of course it's not fact, but it's an educated opinion based on posts on this board and others from fans saying 'I bought multiple copies'.
Not sure why you're getting so defensive about this?