Remove all duplicate word from string using shell script

One more awk, just for fun:

$ a="aaa bbb aaa bbb ccc aaa ddd bbb ccc"
$ echo "$a" | awk '{for (i=1;i<=NF;i++) if (!a[$i]++) printf("%s%s",$i,FS)}{printf("\n")}'
aaa bbb ccc ddd

By the way, even your solution works fine with variables:

$ b="zebra ant spider spider ant zebra ant" 
$ echo "$b" | xargs -n1 | sort -u | xargs
ant spider zebra

With tr, sort and uniq

echo "zebra ant spider spider ant zebra ant" | tr ' ' '\n' | sort -u

echo "zebra ant spider spider ant zebra ant" | tr ' ' '\n' | sort -u | xargs

to get one line

$ echo "zebra ant spider spider ant zebra ant"  | awk -v RS="[ \n]+" '!n[$0]++' 
zebra
ant
spider

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