I have come across a fair number of such cases Wm the elder + Wm the younger etc whilst doing my will summaries - but as you say generally in first half of 18th C tho I havn't done much work yet on 17th C wills - I suspect there might be a link to the English custom of the 17th C and earlier of naming the child after a god-parent (who might well then contribute something towards cost of rearing said child.