How do you register a global module exactly once in NestJS and what happens if you register it twice?
Register a global module only in the root AppModule. Importing the same global module in multiple feature modules causes duplicate provider instantiation, non-deterministic resolution if factories have side effects, and in some NestJS versions silently shadows the first registration with the second.
Correct single registration vs incorrect double registration
Best practices for global module registration:
Register global modules only once in the root AppModule.
Use the isGlobal: true option pattern (used by @nestjs/config) as the idiomatic approach.
Add a comment in AppModule marking global module registrations so teammates know not to import them elsewhere.
If a global module is accidentally imported twice, watch for duplicate provider warnings in startup logs.