She could not live upon the Past — Maid Marian

"Duke or Robin" at the end is suspicious. If Dickinson used Earl instead of Duke, then this riddle would have two clues too close due to the legend of Robin Hood and Earl of Huntingdon. Maid Marian was his wife.

This poem again states Dickinson's own mind, "could not live upon the past, the present did not know her." In this case, duke means "a man of showy demeanour or appearance (OED 4b)" and Robin a commoner.

She could not live upon the Past
The Present did not know her
And so she sought this sweet at last
And nature gently owned her
The mother that has not a knell
for either Duke or Robin
(F.1535/J.1505)
[1] She, the Past:: Matilda Fitzwater, daughter of the Baron of Arlingford, the past of Maid Marian (her name in Sherwood), as one of the legends of Robin Hood.
[2] Present, not know her:: her noble world could not understand her.
[3] sweet at last:: she chose to marry Robin Hood, the outlawed Earl of Huntingdon.
[4] nature, owned her:: Marian joined the merry men of Sherwood; she owned the forest or the nature owned her.
[5] mother:: the nature. not a Knell:: merry, a hint on Merry Men; "we are true men and are merry by nature." (Maid Marian, 1822)
[6]Robin:: the personal name, in more or less allusive or general application (OED 1); a commoner; a hint on Robin Hood. Duke:: a noble; Robin Hood was a noble and commoner.

"May I never again have roof but the blue sky, nor canopy but the green leaves, nor barrier but the forest-bounds; with the foresters to my train, Little John to my page, Friar Tuck to my ghostly adviser, and Robin Hood to my liege lord. I am no longer lady Matilda Fitzwater, of Arlingford Castle, but plain Maid Marian, of Sherwood Forest." ─ Maid Marian (1822) by Thomas Love Peacock