I agree Judith, curiouser and curiouser,
Do we know when he returned to Sydney from VDL? Was it with his parents, or did he travel alone ?
Re the multiple entries at NSW BDM ... suggesting to me that if these are for the same person, that one would be for the original parish register (which could be anywhere in what was then NSW, eg VDL was still part of NSW at that time), and one would be for the transmitted record to the NSW Chaplain (in accordance with Gov Macquarie's General Orders of 1810. The NSW Chaplain was of course the Rector of St Philips, in Sydney and was C of E. But not just C of E records were to be forwarded to him.... any denomination sent records to Rev Cowper, from anywhere in the Colony and in the South Seas....
So, it is quite possible that William Francis Dubock Brown was baptised in the territory that Macquarie governed as Governor of the penal colony of NSW.
I am not sure about the entry for 1817 with no parents names given. I have a similar NSW BDM cert from that era .... (I just had to purchase it, rather than a transcript), it is for a baptism, with NO surname, and NO parents listed .... It is suitably framed and hanging at my front door, to remind anyone that cares to look at it, that you cannot always believe certified information on Official Documents.... The original parish record of course gives ALL the family names.... Yep, NSW and yep, Sydney Town, and during Gov Lachlan Macquarie's reign.
Cheers, JM