What causes "Task %d redefines '%s' from task %d" in hashicorp/nomad?

Rename one of the duplicate task blocks to a unique name within the group.

Task %d redefines '%s' from task %d

Error message

Task %d redefines '%s' from task %d

What it means

TaskGroup.Validate builds a map of task names and rejects duplicates: when a task reuses a name already defined earlier in the same group, this error is appended, referencing the 1-indexed positions of both the duplicate and the original task. Task names must be unique within a task group because they key allocation state, service checks, and log streams.

Source

Thrown at nomad/structs/structs.go:7256

			if err := tg.Migrate.Validate(); err != nil {
				mErr = multierror.Append(mErr, err)
			}
		}
	default:
		if tg.Migrate != nil {
			mErr = multierror.Append(mErr, fmt.Errorf("Job type %q does not allow migrate block", j.Type))
		}
	}

	// Check that there is only one leader task if any
	tasks := make(map[string]int)
	leaderTasks := 0
	mainTasks := 0
	for idx, task := range tg.Tasks {
		if task.Name == "" {
			mErr = multierror.Append(mErr, fmt.Errorf("Task %d missing name", idx+1))
		} else if existing, ok := tasks[task.Name]; ok {
			mErr = multierror.Append(mErr, fmt.Errorf("Task %d redefines '%s' from task %d", idx+1, task.Name, existing+1))
		} else {
			tasks[task.Name] = idx
		}

		if task.Leader {
			leaderTasks++
		}

		if task.IsMain() {
			mainTasks++
		}
	}

	if leaderTasks > 1 {
		mErr = multierror.Append(mErr, fmt.Errorf("Only one task may be marked as leader"))
	}

	// A task group made up entirely of lifecycle tasks (prestart, poststart, or

View on GitHub (pinned to 482b49bf1a)

Solutions

Rename one of the duplicate task blocks to a unique name within the group.
If the tasks are meant to be independent instances, place them in separate groups instead.

Example fix

// before
task "app" { driver = "docker" }
task "app" { driver = "exec" }
// after
task "app" { driver = "docker" }
task "app-sidecar" { driver = "exec" }

Defensive patterns

Strategy: validation

Validate before calling

// Go: enforce unique task names per group
for _, tg := range job.TaskGroups {
  seen := map[string]bool{}
  for _, t := range tg.Tasks {
    if seen[t.Name] {
      return fmt.Errorf("group %q: duplicate task name %q", tg.Name, t.Name)
    }
    seen[t.Name] = true
  }
}

Prevention

Name tasks after copying a block and immediately rename it.
In generators, derive task names from unique loop keys.
Add a unit test that validates all canonical job specs with Nomad's structs API.

When it happens

Trigger: Submitting a job with two `task` blocks sharing the same `name` inside one `group` block.

Common situations: Copy-pasting a task block and forgetting to rename it; merging job fragments; templated loops generating tasks with identical names.

Related errors

AI-assisted analysis of hashicorp/nomad@482b49bf1a (2026-09-04). Data as JSON: /api/errors/74f6a1585fe40749. Report an issue: GitHub.