Print each email exactly once (lowercased), in first-seen order.
Read emails one per line. Print each unique email **lowercased**, preserving first-seen order. Duplicates differ only by case.
Test input:
Foo@x.com
bar@y.com
FOO@x.com
Baz@z.com
bar@y.com
Expected output:
foo@x.com
bar@y.com
baz@z.com
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