Extract the two recorded parents from every matching certificate, and use duplicate removal so that repeated certificates do not repeat an individual. This helper is shared by the remaining Haskell definitions:
unique :: Eq a => [a] -> [a]
unique [] = []
unique (x:xs) = x : unique [y | y <- xs, y /= x]

parents :: Database -> Person -> [Person]
parents db p = unique [q | (baby,mother,father) <- db,
                          baby == p, q <- [mother,father]]
A person need not have a certificate of their own to appear in the output: a parental field is already evidence of the relationship. The resulting list preserves first occurrence order, with mother before father in each certificate. Its set of entries is
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