Output whole line once for each unique value of a column (Bash)

Question

This must surely be a trivial task with awk or otherwise, but it\'s left me scratching my head this morning. I have a file with a format similar to this:

<

Answer 1

I would use Perl for this:

perl -nae 'print unless exists $seen{$F[1]}; undef $seen{$F[1]}' < input.txt

The n switch works line by line with the input, the a switch splits the line into the @F array.