Error: Duplicate output definition: "was already defined at outputs.tf"

TL;DR

Two output blocks share the same name. Outputs must be unique per module, just like resources. Delete or rename one of them, then terraform validate.

The error

Error: Duplicate output definition
An output named "lambda_execution_role_arn" was already defined at outputs.tf:3,1-35.

Steps to fix

1. Find all definitions: grep -n 'output "lambda_execution_role_arn"' outputs.tf (use your output name).

   • Expected: two or more output blocks with the same name, often a leftover single-line version plus a new block-style one.

2. Keep one definition and delete the other. If both carry different values, decide which one callers should see and merge.

   • Expected: exactly one output "[name]" block remains.

3. Run terraform validate.

   • Expected: Success! The configuration is valid.

When to use this

• terraform validate or plan fails with Duplicate output definition after merging branches, refactoring outputs.tf, or converting output styles.

When NOT to use this

• Duplicate resource configuration is the same idea for resources (fix the same way). Unsupported attribute on module.[name].[output] means the caller wants an output the module never defined.

Compatibility

• All Terraform versions; outputs have always required unique names per module.

Root cause

Output names form the module's public interface, so each name must map to exactly one value. Duplicates usually come from refactoring (adding a block-style output without removing the old single-line one) or bad merges that concatenate two versions of outputs.tf.

Edge cases

• Outputs in different modules may share names freely; only duplicates within one module error.
• override.tf files can intentionally redefine outputs; that is the one sanctioned exception, and it still requires the _override.tf filename.