well, not that there is much proof, but they both have black hair, and in the drum island arc, the old lady says that his name was really Gol D Rogers, and D appears to be luffies last name considering that its the only name part he and ace have in common. also, the lady on drum island said " so it looks like the D line continues" which does inply that they are in some way related.
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we already have like a billion topics like this. use the search button
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Yeah, in fact one's right on this page. go here: http://ap.animemedia.org/index.php?showtopic=1214