Why aren't my include guards preventing recursive inclusion and multiple symbol definitions?

Question

Two common questions about include guards:

1. FIRST QUESTION:

   Why aren\'t include guards protecting my header files from mutua

Answer 1

fiorentinoing's answer is echoed in Git 2.24 (Q4 2019), where a similar code cleanup is taking place in the Git codebase.

See commit 2fe4439 (03 Oct 2019) by René Scharfe (rscharfe).
^{(Merged by Junio C Hamano -- gitster -- in commit a4c5d9f, 11 Oct 2019)}

treewide: remove duplicate #include directives

Found with:

git grep '^#include ' '*.c' | sort | uniq -d