test$ nasm -v
NASM version 2.16.02rc2 compiled on Oct 12 2023
test$ cat test1.asm
nop
label:
extern label
test$ cat test2.asm
nop
extern label
label:
extern label
test$ cat test3.asm
label:
extern label
test$ nasm -f obj test1.asm
test1.asm:3: error: label `label' inconsistently redefined
test1.asm:2: info: label `label' originally defined here
test$ nasm -f obj test2.asm
test$ nasm -f obj test3.asm
test$