For example, take
Solution #1 [0 sec]:
1 2 15 16
12 14 3 5
13 7 10 4
8 11 6 9
and see if you find it's mirror around the diagonal (rot 90, mirror) in the first 880 found from the exhaustive search.
...
Solution #222 [11 sec]:
1 12 13 8
2 14 7 11
15 3 10 6
16 5 4 9
You can't use the first 880 solutions generated by this code as the unique set to reflect and rotate.