There are 11 combinations of using those checkboxes. Try all 11 before giving up. Usually one will work. Checking both "a" and "b" - don't do that as that is identical to checking only "b" (or only "a" - I forget - just don't check both).
If none of that works you can try editing it using this procedure (meshlab is free):
http://meshlabstuff.blogspot.com/2009/04/how-to-remove-internal-faces-with.html
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illuminarti 18
In Cura, 'Expert' -> 'Open Expert Settings...', try turning on 'Combine Everything (Type A)'. If that doesn't work, try the other options.
I didn't try slicing it, but I suspect the two parts aren't properly unioned together in the STL file, but rather exist as two separate intersecting objects. That confuses the slicers, as they (correctly) assume that the bit that is inside two separate walls should be considered to be on the outside of the shape. The various 'Fix Horrible' settings try to work around those issues.
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