For type definition, you must consider each ".c" file as autonomous :

1. (structs.c + structs.h) -> has only one definition of Point. It produces structs.o
2. (main.c + structs.h) -> has only one definition of Point. It produces main.o

If you manually add a definition of Point directly in main.c, you'll have 2 definition of Point in step 2, which leads to the expected error.

Type definition are not kept in ".o" files, so there's no problem of duplicate definition at linking stage.