Two global structures were defined under the same name in different components. No error was raised by linker.
Checking default options:
gcc -Q --help=common
shows that -fcommon is enabled.
From gcc-arm-none-eabi-9-2020-q2-update/share/doc/gcc-arm-none-eabi/info/gcc.info:
-fno-common
'-fno-common' In C code, this option controls the placement of global variables defined without an initializer, known as "tentative definitions" in the C standard. Tentative definitions are distinct from declarations of a variable with the 'extern' keyword, which do not allocate storage.
Unix C compilers have traditionally allocated storage for uninitialized global variables in a common block. This allows the linker to resolve all tentative definitions of the same variable in different compilation units to the same object, or to a non-tentative definition. This is the behavior specified by '-fcommon', and is the default for GCC on most targets. On the other hand, this behavior is not required by ISO C, and on some targets may carry a speed or code size penalty on variable references. The '-fno-common' option specifies that the compiler should instead place uninitialized global variables in the BSS section of the object file. This inhibits the merging of tentative definitions by the linker so you get a multiple-definition error if the same variable is defined in more than one compilation unit. Compiling with '-fno-common' is useful on targets for which it provides better performance, or if you wish to verify that the program will work on other systems that always treat uninitialized variable definitions this way.