Mark Hamill is credited with two roles in The Last Jedi: Luke Skywalker (obviously), and someone called Dobbu Scay
Who is Dobbu Scay?
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Pablo Hidalgo has confirmed in a twit that the short drunk alien who tried to play BB-8 like a slot machine was Mark Hamill's cameo, Dobbu Scay
Q: Going into viewing number 6 of #thelastjedi so I'm begging @pablohidalgo or @rianjohnson to tell me the name of the Canto Bight patron who plays BB-8 like a slot machine. Inquiring minds want, nay, NEED to know.
@PabloHidalgo: I'll just say this - he's listed in the credits as the second role [that] someone very prominent in the cast plays. :)
I initially thought this might be Warwick Davies' cameo but this is the only second role by any main cast character.
Further confirmation (from Pablo Hidalgo) can be found here.