'sort_field=$47; $47=""; !seen[$0]++ { print $0 " " sort_field }'

$ cat file
1 2 3 4
5 6 7 8
1 2 3 x
a b c d
5 6 7 y

awk '
    {
        line = $0     # remember the original state of this line
        NF--          # forget about the last field
    }
    !seen[$0]++ {print line}   # if the "new" line is unique, print the "old" line
' file

1 2 3 4
5 6 7 8
a b c d

sort -uk1,46 infile.txt