The pkg-config(1) mentions that for --cflags and --libs, "Flags are "compressed" so that each identical flag appears only once.".

This "compression" sounds harmless but it really isn't. For example, in one real-life ".pc" file we had the following:

Libs: -L/home/sothy/seastar/build/release -Wl,--whole-archive -lseastar -Wl,--no-whole-archive -g -Wl,--no-as-needed   -fvisibility=hidden -pthread  -laio -lboost_program_options -lboost_system -lstdc++ -lm -lboost_unit_test_framework -lboost_thread -lcryptopp -lrt -lxenstore -L/home/sothy/seastar/build/dpdk/lib -Wl,--whole-archive -lrte_pmd_vmxnet3_uio -lrte_pmd_i40e -lrte_pmd_ixgbe -lrte_pmd_e1000 -lrte_pmd_ring -Wl,--no-whole-archive -lrte_hash -lrte_kvargs -lrte_mbuf -lethdev -lrte_eal -lrte_malloc -lrte_mempool -lrte_ring -lrte_cmdline -lrte_cfgfile -lrt -lm -ldl -lhwloc -lnuma -lpciaccess -lxml2 -lz 


Note how the "-Wl,--whole-archive" option (and also "-Wl,--no-whole-archive") appears twice, surrounding two different sets of libraries. Dropping one of the copies of these flags, or reordering them, is absolutely wrong because their exact position matters.

I can easily work around this bug - e.g., by moving libraries around so we have only one whole-archive/no-whole-archive pair, or by changing the multi-word "-Wl,--whole-archive -lseastar -Wl,--no-whole-archive" to a single word "-Wl,--whole-archive,-lseastar,--no-whole-archive". 

However, I'd prefer not to work around this bug - other people might encounter it in other scenarios as well. There are various other cases where repeating the same flag or library again can cause the compilation to change. so why not just fix it? Why is it important that pkg-config "compress" identical flags? Where did this requirement come from?