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On screen only, it has happened on at least these occasions:

"Day of the Daleks" (Third Doctor met himself)
"The Three Doctors" (1, 2, 3)
"The Five Doctors" (1, 2, 3, 5, cameo by 4 in stock footage)
"The Two Doctors" (2, 6)
"Father's Day" (Ninth Doctor briefly saw himself)
"Time Crash" minisode (5, 10)
"The Big Bang" (Eleventh Doctor met himself)
"Last Night" (DVD minisode: Eleventh Doctor Met himself)
"Space"/"Time" (Comic Relief minisode: Eleventh Doctor met himself)
"The Name of the Doctor" (1-11 plus War, if being within his own time stream counts)
"The Day of the Doctor" (War, 10, 11 and implied all the others off-screen up to 12, plus Curator)

If one wants to be literal, "Flatline" can be counted as Clara impersonates/stands in for the Doctor briefly. But not "Death in Heaven" as Clara's bluff does not play out in front of the Doctor.