Voilá! No more unintended ligatures.
Apparently I'd been tapping into font files left over from previous operating systems. The fonts left in place by the "Resolve Duplicates" command are all dfonts.
Thanks, Winfried!
Following the advice from Winfried Huslik, I opened Font Book,
selected all the fonts that had dots after them, went to the Edit
menu, selected "Resolve Duplicates", shut Font Book, and restarted
the computer.
