gcc -g -Wall -Wextra -o main main.o sub.o
sub.o: In function `sub2':
sub.c:(.text+0x7): multiple definition of `sub2'
main.o:main.c:(.text+0x0): first defined here
collect2: error: ld returned 1 exit status
Makefile:4: recipe for target 'main' failed
make: *** [main] Error 1
