Remove line if field is duplicate

If you're open to using Perl:

perl -ane 'print if ! $a{$F[0]}++' file

-a autosplits the line into the @F array, which is indexed starting at 0
The %a hash remembers if the first field has already been seen

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This related solution assumes your field separator is a comma, rather than whitespace

perl -F, -ane 'print if ! $a{$F[0]}++' file

awk '{ if (a[$1]++ == 0) print $0; }' "$@"

This is a standard (very simple) use for associative arrays.

this is how to remove duplicates

awk '!_[$1]++' file