In the case of the name, John, for example, there is also the matter of
there being numerous saints of that name including, St John the
Baptist, St John the Evangelist, St John Chrysostom, etc.
Theoretically two brothers of the same forename (Christian name) could
then have two separate saints' days (name days) and, therefore, in
theory, at least notionally, two distinct Christian names were being
credited/honoured even though which particular dedicatory saint each
son is named for is not recorded.
The same of course applies to other male saints as well as
dedicatory names given to females.
This echoes the possibility of doubling up on godparent-derived names
(as Matt Tompkins mentioned) as this could be a case of layering of
honour/dedication with a given saint's name being honoured along with
a godparent's (whose particular dedicatory saint's name was perhaps
being carried forward to another generation).
I suppose one could consider this a form of the promotion of the cult
of a given saint, and the reinforcement of one's family ties to that
holy person with the side benefit of including a godparent in the baptised's
support/honour system.
Not all saints names given to children need necessarily have been
derived from a godparent of the same sex. Indeed, one sees in the case
of one key ERNLE forebear, Michael ERNLE, of Bourton manor, Bishop's
Cannings, Wilts. (d. ca 1593-4), that he was baptised on the 29 September
1541, i.e. the feast day of St Michael the Archangel (which was perhaps also
the day on which he was born as he was eldest son and heir of his
father John ERNLE, gent.), according to the parish register of his
mother's home parish,
as she was a HYDE of Denchworth, in Berkshire, viz.:
http://archive.org/stream/registersofdench00denc#page/n9/mode/2up
N.B. The n-less spelling of the surname recorded in that baptismal
entry is just one of the sometimes confusing (and thus highly
missable) spelling variations under which I have found members of the
Sussex and Wiltshire ERNLE sib recorded in the period during which the
ERNLE surname flourished (ca 1166-1787) before dying out (apparently)
in the male line.
Richard Carruthers