Alas, there is no "Auto Complete" in 15.2, it is "Auto-complete parentheses and braces"
However, I don't think it's JMP -- it's this keyboard I was given.
It other applications -- including this website:
Type " once, and it doesn't appear. So you type " again, and you get "".
Typing " followed by a space, and you get "
Now the JMP JSL editor appears more hypersensitive to whatever the keyboard is doing, as just typing " once will result two "" immediately.
I tried reducing the repeat delay and repeat rate in Windows for the keyboard -- no joy.