xdg had some valid questions, but barring additional info, here's a one-liner (see perlrun) solution:

perl -i -0777 -pe 's/(\bAAAA )(QQQQ)(PPPP)( BBBB\b.*?\bCCCC XXXX )\2\3
+\b/$1$2$3$4$2/sg' /tmp/f
[download]

Tested against this data:

blah
stuff AAAA QQQQPPPP BBBB stuff
more stuff blah
and CCCC XXXX QQQQPPPP and more
stuff.
blah
stuff AAAA QQQQPPPP BBBB stuff
more stuff blah
and CCCC XXXX QQQQPPPP and more
stuff.
[download]