[goodside]

This is known as “negative concord” and shows up often in colloquial English. While popularly associated with African American and Appalachian dialects, it actually occurs almost everywhere English is spoken. (E.g. Pink Floyd’s chorus, “We don’t need no education / We don’t need no thought control”.)

Even in formal writing, double negatives are common using “nor”. Consider the sentence, “He is not satisfied at all with the recommendations of Mr. Trump, nor with those of Mrs. Clinton.” If you change “nor” to “or”, the meaning remains identical. This shows the negative “nor” reiterates rather inverts the prior negation. Note the “nor” version is no less formal or professional in tone because of its double negative.

Negative concord even has precedent in Shakespeare: “I never was, nor never will be” from Richard III.

Most would suggest avoiding negative concord in a formal writing (excepting “nor”), but colloquially it’s been part of the language for hundreds of years. In a conversational context, the line you quoted is standard English with or without the “don’t”.

[squiggleblaz]

That sounds like a misplaced and ideological canned response, since I've never heard negative concord in this context.

Most people would just accept it as getting confused about the number of negatives in a sentence - as indeed did the op.

Not everything is simply informal grammar; real fleshy human beings do get confused when they try complicated sentences like double negatives.

[goodside]

> This sounds like a misplaced and ideological canned response, since I’ve never heard negative concord in this context.

You have a misplaced trust in your instincts. The following examples of this exact double-negative construction (“...wouldn’t be surprised if ... didn’t...”) are quoted from newspapers and BBC specials:

> But the seeds have been sown and I wouldn't be surprised if she didn't try, once she's feeling more confident herself, to persuade him into the deep end.

> "I wouldn't be surprised if we didn't see there are a couple of days with some good news and very, very positive market news," Houge said, noting potential days of 10 percent spikes.

> "I wouldn't be surprised if you didn't lose $157,000 in taxes," Van Tuinen said.

> It was late, and I wouldn't be surprised if he didn't leave some of his audience back down the road somewhere impaled on point number 10 or 11.

Here’s a quote of John F. Kennedy using this construction in a recorded interview:

> President Kennedy: That's what I think. I would have been impeached. I think they would have moved to impeach. I wouldn't be surprised if they didn't move to impeach right after this election, on the grounds that I said … and didn't do it … and let … I mean, I'd be …

I Googled these examples in five seconds. You could have too, but instead you chose to lob insults and embarrass yourself.