Detected by a simple Shell script:

for i in $(git ls-files -- '*.[ch]'); do
    awk 'BEGIN {
        fail=0
    }
    /# *include.*\.h/{
        match($0, /["<][^">]*[">]/)
        arr[substr($0, RSTART+1, RLENGTH-2)]++
    }
    END {
        for (key in arr) {
            if (arr[key] > 1) {
                fail=1
                printf("%d %s\n", arr[key], key)
            }
        }
        if (fail == 1)
            exit 1
    }' $i

    if test $? != 0; then
        echo "Duplicate header(s) in $i"
    fi
done;

A later patch will add the syntax-check to avoid duplicate
headers.