Повторяющиеся слова в строке

echo "windows xp windows linux debian linux"  |awk 'BEGIN {ORS=" "} {for (i=1;i<=NF;i++) if (h[$i]==0) {h[$i]=1; print $i}}'

echo "windows xp windows linux debian linux" \
| awk '{ for(i=1;i<=NF;i++){a[$i]++} }END{ for(i in a){if(a[i]==1){printf("%s ",i)}} }'

echo "windows xp windows linux debian linux" \
| awk '{ for(i=1;i<=NF;i++){a[$i]++} }END{ for(i in a){printf("%s ",i)} }'

$ declare -A a;for i in windows xp windows linux debian linux;do a[$i]=;done
$ echo "${!a[@]}"
xp debian windows linux

$ echo "windows xp windows linux debian linux" | sed -r 's/(\<\w+\>)(.*)\1/\2/g'
 xp   debian 

$ echo "a a c a a d a a f a f" |\
sed -r 's/(\<\w+\>)(.*)\1(.*)/\1\2\1\3\n\1/;T;:l;s/(.*)(.*)\n\1$/\2\n\1/;tl;s/\n.*//'
  c   d   f  f

$ cat test.sh 
#!/bin/bash

a="windows xp windows linux debian linux";
b="";
for i in ${a};
do
        COUNT=0;
        for j in ${a};
        do
                if [ "${i}" == "${j}" ];
                then
                        (( COUNT++ ));
                fi
        done

        if [ ${COUNT} -eq 1 ];
        then
                if [ "${b}" != "" ];
                then
                        b="${b} ";
                fi
                b="${b}${i}";
        fi
done

echo "\"${a}\" -> \"${b}\"";

$ ./test.sh 
"windows xp windows linux debian linux" -> "xp debian"