Code doesnt compile from chapter 4 and beyond

[CUEngineer]

This code stops compiling after chapter 4, have tried by writing my own src code from scratch following the book and by directly compiling the provided source code, both fail the same way.

"
ld: scheduler.elf:(.bss+0x48): multiple definition of curr_sch_pid'; kernel.elf:(.bss+0x68): first defined here ld: scheduler.elf:(.bss+0x4c): multiple definition of next_process'; kernel.elf:(.bss+0x6c): first defined here
ld: warning: starter.o: missing .note.GNU-stack section implies executable stack
ld: NOTE: This behaviour is deprecated and will be removed in a future version of the linker
ld: warning: 539kernel.elf has a LOAD segment with RWX permissions
make: *** [Makefile:16: build] Error 1
"