Dilemma vs. dilemna

This is a mistake I've made in the past, explained at World Wide Words:

There is no doubt about the correct spelling: the word is Greek, from di-, twice, plus lemma, a premise. It has always been spelled that way, at least according to the dictionaries that I’ve consulted, ancient and modern (it dates from the sixteenth century as a term in rhetoric)...

The ["dilemna"] spelling is certainly rife today. It’s easy to find thousands of examples by searching newspaper and book archives. Of these, a large number, certainly a significant majority, are misprints or simple errors. The reason for it seems to be a mental confusion with other words in English that are spelled with mn but said as mm, including autumn, hymn, condemn, solemn and column.

A search of historical literature shows that in earlier times it was quite common and turned up in works by well-known authors. These are a few eighteenth-century examples:

In this Dilemna, as I was very pensive, I stept into the Cabin, and sat me down. (Robinson Crusoe, by Daniel Defoe, 1719.)

The nation saw themselves reduced to a ridiculous dilemna upon their testimony. (The History of England, by Oliver Goldsmith, Vol 3, 1771.)

There was a famous ancient Instance of this Case, wherein a Dilemna was retorted. (Logick, by Isaac Watts, 1772.)

It’s not just in English that the problem is known. In French it sometimes appears as dilemne instead of dilemme. Native French speakers have reported that they, too, were taught the wrong form...

In view of the very large number of historical examples, it makes me wonder if the variant spelling has persisted in the language for many generations, unnoticed by dictionary makers or repeatedly dismissed as a simple error.