Unknown vimrc line: %!awk

I haven't used Vim in a while and when I started using it again I noticed this line in $MYVIMRC

:nnoremap <f3> :%!awk '!a[$0]{a[$0]=1;print}'<bar>noh<cr>

I understand that it's mapping to the F3 key, but I can't figure out what the rest of the line does. Can anyone tell me?

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