If I were to discover that I had an identical twin , first of all i'd try to determine if I have any special qualities worth cloning   ( because you can never truly tell until you do a quick introspection ) once i'd obviously rule that one out , i'd continue with a 2 hour long spiral of confusion .

After cognitive abilities return to normal , i'd have a REEEEALY long chat with my parents , which would undoubtably lead to a  2 month long subscription to the league of silent treatments,  a truly minor inconvenience in search of the truth !!! 

Furthermore ruling out kidnapping , abductions , halucinations , drug induced coma dream ( hard one tho ;o), I'l begin cherishing every moment i'd have with him , because for all I know it may be the last . 

Also , for all intents and purposes he would be an excelent candidate for co-op video games , and i'll finaly have someone to play monopoly with ;) !

Hopefully it will be me from the future.

but is that sex, or masturbation?

also i would play videogames, be my own workout buddy because i need one, and take turns going to work so we can both be lazy

Finally the threesome I've always wanted

If I had an identical twin we would both be bummed

if it's good enough for the lannisters...