^ Basically what DuKe™ said.
-Don't get grabbed. Marth's grabs **** himself (lol)
-Dashdance to make him whiff attacks, then grab him (and proceed to ****).
-I'm not sure about this one, but I'd advise you to use your ftilt a little more then usual, since I BELIEVE it outranges Marth's aerials. Use it for spacing.
-If ether Marth is put off the stage, that's a stock. Don't be the first one off the edge.
-Be aware that all that information can be used against you.
In general, Marth dittos are all about who can get the first grab off. Once Marth gets a grab on another Marth, that's a combo. Also, be aware that you don't ourange your opponent in a Marth ditto (which is why I suggested you use the attack with the most range Marth has. Remember that he can use this same attack against you. Don't underestimate how far it reaches out). As I said too, Marth's absurd edgeguarding game paired with his mediocre recovery means that, if a Marth is put off the stage, he is simply not comming back, period. Get your opponent off the stage ASAP (like 40% if you can), and edgeguard.
EDIT: Forgot the most important piece of advice for Marth dittos. Get all women, young children and elderly out of the room, otherwise the sheer amount of cheap a Marth ditto radiates will overwhelm them.