My question concerns "Pinga" and (double) negation. An example you used is "Pinga ou fè sa!" Is this different from "Pinga ou pa fè sa!"

Yes.  It's definitely different.
I guess a double negation comes thru as a positive sentence. ( I don't know how it works in English)

Pinga ou fè sa!
Don't do that!
Don't you do that!
Pinga in this sentence comes off  as a warning not to do something

Pinga ou pa fè sa non! comes through as you better do it, you should do it, I hope you do it
My emphatic "non" at the end of that sentence doesn't mean anything, but if I were to say that sentence,  that NON would be there.
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