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echo "4 2 d c b 1 a b A b 1 B C a b c d" | ./clean.awk

# output
a b c d 1 2 4

clean.awk

#!/usr/bin/awk -f
{
for(i=1;i<=NF;i++)
a[tolower($i)]=1
}

END{
for(i in a)
...