Error: Duplicate object key: "Two different items produced the key" in a for expression

TL;DR

Your { for ... : key [your value] value } produced the same key twice. Terraform refuses to guess which value wins. Either make the keys unique, or add the grouping ellipsis (...) after the value expression to collect duplicates into lists.

The error

Error: Duplicate object key

  on .terraform/modules/repository/main.tf line 404, in locals:
 398:   teams = { for i in concat(
 399:     local.team_admin,
 400:     local.team_push,
 401:     local.team_pull,
 402:     local.team_triage,
 403:     local.team_maintain,
 404:   ) : i.slug => i }

Two different items produced the key "nios-build-write" in this 'for' expression.

Steps to fix

1. Read which key collided (here: nios-build-write). Find the two items that produce it.

   • Expected: you can point at both source items.

2. Decide: should duplicates merge or group? If one should win, restructure so keys are unique (e.g. merge() several single-key maps, later arguments winning), or key by a composite like "${i.slug}-${i.permission}". If duplicates are legitimate, enable grouping:

   teams = { for i in local.all_teams : i.slug => i... }

   • Expected: each key maps to a list of values; downstream code indexes into lists.

3. Run terraform validate, then plan and check the resulting map.

   • Expected: no duplicate-key error; the map shape matches what consumers expect.

When to use this

• plan/validate fails with Duplicate object key in a for expression building a map, usually after concatenating lists that can share keys.

When NOT to use this

• Duplicate resource configuration is about resource blocks, not map keys. If the collision is unexpected, grouping with ... hides a data bug; prefer making keys unique.

Compatibility

• Terraform 0.12+ for expressions; the grouping ellipsis is available in all 1.x versions.

Root cause

Object construction requires unique keys. When two iterations emit the same key, Terraform cannot pick a value without silently dropping data, so it errors. The ... ellipsis opts into the explicit alternative: group values per key into a list.

Edge cases

• for_each over the resulting map with grouped values gives you lists per key; adjust each.value handling.
• The ellipsis only works when building objects, not tuples: [for ... : k...] errors.
• distinct() on the input list before the for expression is another way to force uniqueness when duplicates are noise.

mineiros-io/terraform-github-repository#180 is a real instance: terraform plan fails building the teams local with "Error: Duplicate object key ... Two different items produced the key \"nios-build-write\" in this 'for' expression." The cause was the same team slug appearing in more than one permission list (teamadmin, teampush, teampull, teamtriage, team_maintain) while the concat + for pattern does not tolerate duplicate keys. The proposed fix replaces it with merge() of individual maps ordered by ascending privilege so the highest permission wins. The error message itself suggests the ellipsis (...) grouping alternative.