"Hey, what do I know? I was dead before mystery twin decided he was Sherlock Holmes, apparently."

His money is totally on clone. The only question is whether Tony managed it before or after.

Tony managing to pull a previously unseen twin brother out of his ass and convince the public that his parents just forgot is a depressingly plausible scenario. So, for that matter, is his managing to hide a clone from Howard for God knows how long. He could've raised an army of them in that junk-filled workshop of his and Howard never would've noticed.