Shell Programming and Scripting

awk '!($1 in T) {T[$1]=$0; next} {T[$1]=""} END {for (t in T) if (T[t]) print T[t]}' file
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awk '
!($1 in T)      {T[$1]=$0               # on the first occurrence of $1, save the line in T array
                 next                   # stop processing this line
                }
                {T[$1]=""               # any further occurrence, set T element to empty (NOT delete!)
                }
END             {for (t in T) if (T[t]) # iterate through T arr; if a non-empty value found:
                        print T[t]      # print it
                }
' file

awk '!C[$1]++{A[$1]=$0; next} {delete A[$1]} END{for(i in A) print A[i]}' file