Rebuild a rectangular array from a corner

Canvas, 1 byte

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Outputs as a multiline string

Haskell, 25 24 bytes

r=(++)<*>reverse
r.map r

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Python 3, 38 bytes

lambda a:[b+b[::-1]for b in a+a[::-1]]

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Takes a list of lists and returns a list of lists.

Explanation:

lambda a:                              # anonymous lambda function
                   for b in a+a[::-1]  # for each row in the array and the upside-down array
          b+b[::-1]                    # the row with its reverse appended
         [                           ] # return in a list