[clang] Add check for duplicates to make_cxx_dr_status script (#67969)

While working on #67965, I stumbled upon the fact that `make_cxx_dr_status` script doesn't check for duplicated comment (last comment wins), so I added this check.

It even found another instance of duplicated comment: CWG1223 was marked as CWG1227. I fixed status of the former.