If by “scheduled” you mean “scheduled before the season started”, I don’t know the answer–I suspect it was some time during the mid-1980’s. Nowadays all double-headers are make-up double-headers.
Among make-up double-headers, a growing percentage, but not all, are day-night. Make-up double-headers which have to be played on short notice (rain the previous day) in cities with low attendance are more likely to be conventional. One such was played on September 13, 2006 by Milwaukee at Pittsburgh.
According to ESPN, Cleveland has a DH scheduled at Seattle, to make up one of the Snow Bowl games, at 3:05 and 6:35 on September 26. I assume that this will be two games for one admission. But again, by “scheduled” they mean “scheduled to make up a postponement”, not “scheduled before the season started”.