Richardson states that Francis West of Virginia had a son Francis. Francis West of Virginia married his first wife before 1625/6. He would have been 40 years old in 1626 so presumably his marriage could have been as early as 1606 when he was 20 years old, a common age to marry at that time. The 1606 birth date for Francis of Duxbury is approximate and there are no known records of his birth.
Francis of Virginia left a will that was dated 17 December, 1629[8] and in it he refers to his son Francis as being under 21 years, making his son's birth year after about 1608, not that different from the 1606 approximate birth date of Francis of Duxbury.
Just making these points to show that, timeline-wise, it is entirely possibly for Francis of Duxbury to have been the son of Francis of Virginia, perhaps born out of wedlock.