The earliest instance of "cannot help but"—in the modern idiomatic sense—that I've been able to find through Google Books searches is from "Dreams," an essay included in Miscellanies Upon Various Subjects by John Aubrey. Miscellanies was first published in 1696, but it is not clear how much earlier the pieces included in it were written. Aubrey died at the age of 72 in 1697. Here is the relevant text from his essay, discussing a famous dream that the Roman physician Galen had:
Gallen's three Dreams. The third more worthy of being called a Miracle, was, when being twice admonished in his Sleep to cut the Artery that lies between the Fore-Finger and the Thumb, and doing it accordingly, he was freed from a Daily Pain with which he was afflicted in that Part, where the Liver is join'd to the Midriff ; and this he has testified at the end of his Book of Venesection. 'Tis certainly a very great Example, when a Man so great as he was in the Medicinal Art, put so much confidence in a Dream as to try Experiments upon himself ; where be was to run the risque of his Life, in his own very Art. I cannot help but admire his Probity, in the next Place, that where he might have arrogated the Merit of the Invention to himself, and placed it wholly to the Account of the Subtilty and Penetration of his own Genius, he attributed it to God, to whom it was due. In this alone did the Man well deserve to purchase an Immortality to his Name and his Writings.
Also, from a letter written in 1722, reproduced in Calendar of State Papers: Colonial Series: America & West Indies, 1722–1723 (1934) [combined snippets]:
I have frequently laid matters of the greatest importance to the welfare of this Colony before your Lordships and the Secretary of State but have never been honoured with any answers to them and it is very surprizing that tho' I named Mr. Beckford to your Lordships as the cheife instrument of all our divisions and animosities and that he has as well in former Governments as mine constantly opposed whatever has been recomended for the publick service, should meet with such countenance from home as not only to be continued in his post of Comptroller of the Customs but he gives out that he has had sent him copys of my letters which I wrote to your Lordships about him. And I cannot help but observing to your Lordships that of all the gentlemen I ever recomended to be of the Council not one has been put in and those whom your Lordships have thought fit to recommend since I left England live in remote parts of the countrey and at so great a distance that it is with the greatest difficulty imaginable (let the urgency be never so great) to get a quorum together to do business.
And from Ran. Darwall, "Polly Wright: A Birth-Day Rhapsody," in The Lady's Magazine; Or, Entertaining Companion for the Fair Sex (June 1776):
And, as for any simple sonnet,
For her they've such a great regard,
That they're amaz'd–"O! fye upon it!
"She sure deserves a better bard."
And so she does, in humble fashion,
I really cannot help but own :
For her, however, my amorous passion
In rhyme shall now again be shewn.
It thus appears that "cannot help but" has been in use in English in essentially the same idiomatic sense that it has today for more than 300 years.