Well Roy .... this are confusun'!

I have copied your list of children below and will insert my comments.
1. James bapt. 07 April 1773.
Yes - both parents names recorded
2. Joan bapt. 02 October 1774.
Yes - both parents names recorded
3. Thomas bapt. 29 January 1775.
NO - he is actually son of John Tonkin
4. Thomas bapt. 16 May 1776.
Yes - both parents names recorded
5. Peter bapt. 09 August 1778.
Maybe - only father's name recorded
*. Henry bp. 29th July 1781
Maybe - only father's name recorded (Js.)
*. William bp. 19th October 1783
Maybe - only father's name recorded (Js.)
*. Henry bp. 30th January 1785
Maybe - only father's name recorded (son of Js. Tonkin, Bosfrankn)
6. Mary Doble bapt. 25 December 1785.
Maybe - only father's name recorded
7. William bapt. 09 December 1787.
Yes - both parents names recorded
8. Sarah bapt. 18 May 1793.
Yes - both parents names recorded
9. Thomas bapt. 03 March 1797.
Yes - both parents names recorded
So you can remove the first Thomas as his father is clearly recorded as John and then you have three others to consider between Peter (1778) and Mary Doble (1785).
Don't be concerned about the two in 1785 as one was in January and the other in December.
I have a couple of problems with this lot:-
1. the number of children with only the father's name recorded
2. the fact I cannot find burials for the first Henry or the first William. There are three possible burials for the first Thomas but recorded only as 'Thomas Tonkin'.
3. the second Henry is recorded as 'son of James Tonkin of Bosfranken' which is a possible indication of a second James.
And then we can throw in this:-
Mary daughter of Js. Tonkin, als. unkle Hugh baptised 1st January 1783
I think you might have a little more work to do on this one.

CT