[Haskell-beginners] take until duplicate

[mike h]

I’m looking at how to take from a list until a duplicate is found.
e.g.

takeUntilDup [1,2,3,4,3] = [1,2,3,4]

I found this implementation 

takeUntilDup  = foldr (\x r -> x : takeWhile (/= x) r) []

It works but I can’t see how!!? 
The accumulated result is built up in r so with input  [1,2,3,4,3] then,  at the point when r = [1, 2, 3, 4],  the fold is about to use the number 3. i.e. it does takeWhile (/=3) [1,2,3,4]  which gives [1,2] 

Please, how does this work?

Thanks
Mike
.