In Practical English Usage section 370.7, Michael Swan writes:
extra negative in expressions of doubt
In informal standard spoken English, a negative verb (without a negative meaning) is sometimes used after expressions of doubt or uncertainty.
He gives two examples:
- I shouldn't be surprised if they didn't get married soon. (= . . . if they got married soon.)
- I wonder whether I oughtn't to go and see a doctor – I'm feeling a bit funny. (= . . . whether I ought to . . .)
In the first example, does the speaker mean she will get married soon or not?