If you run multiple instances, each one will execute the scheduled job, causing duplicates. Avoid it with leader election, a distributed lock (carefully), a single dedicated scheduler instance, or moving scheduling to an external system.
Expanding on the short answer — what usually matters in practice:
A tiny example (an explanation template):
// Example: discuss trade-offs for "`@scheduled`-in-a-cluster-—-what-can-go-wrong-an"
function explain() {
// Start from the core idea:
// If you run multiple instances, each one will execute the scheduled job, causing duplicates
}