OK, I seem to have finally solved this!
I know I said that before and it turned out to just be another glitch.
This time:
- continued on from the data file above where had the duplicated points and couldn't edit Schedule A directly
- instead, I found I could edit the 'Mortgage Interest Limited Smart Worksheet' on the form 'Tax & Int Wks'
- there, I corrected entry 'A1' to the correct figure (the adjusted 1098 interest, which matches the figure which was there minus the figure on 'C2')
- now, Schedule A figures 8a and 8c are correct!
