As a domain registrar we have had this happen to us in the past, I know that this practice has been used for a long time. It's hard to defend against, but it is possible.
Without the cooperation of both registrars and the previous registrant, nobody will ever know what really happened. Sometimes deals just go bad and the previous registrant claims he didn't sell the domain even though he has the money. Sometimes a domain is transferred and somebody writes a bad check.
Personally, I don't believe either side ever. Because both of them are telling their own truth, unfortunately at some point you have to make a decision on who is more right and who is more wrong.
In the end the decision will be made between Godaddy and Network Solutions transfer teams, it won't be decided here unless the previous owner decides to bring the matter to court. I have personal experience with both of them and I'd bet $50,000 on what will happen, but somebody would lose and it wouldn't be me.
And one more thing if NetSol can't find the previous owner it doesn't matter, it's kind of like winning a summary judgement.
Have fun!
Donny