Hi Peter

I have never researched the Colvin Family , just helping out.
Can confirm twins Margaret and Susan from LDS IGI and Lawsons IOM Burials.

The second baptism of Ann in 1839 is real,as you can now see the original records, but she is not on 1841 census with them, 1834 Ann is. I think your suggestion of a second baptism for some strange reason is right.

Having said that , I have seen some families with same name children, particularly in Scotland. Reason is that a child named after a grandparent may not have been expected to live, so a second child was also named after the GP, but then the first one did live.
I could not find what happened to Ann Quayle/Colvin , no remarriage, or burial..
Donna has you back another generation to Arthur Colvin, so follow up with Donna.
Cheers
David