The most interesting part of this is the claim that the first to use it was a Scots preacher named Duncan. But remember, the argument had a Latin name: aut deus aut homo malus. Something invented in 1870 usually doesn't get a Latin name. So we've got to do better than that.
The earliest use of this approach was possibly by the Scots preacher "Rabbi" John Duncan (1796-1870), quoted in 1870 as a saying used by him during his preaching career: