Error: Duplicate resource "aws_vpc" configuration

Fix terraform 'Error: Duplicate resource "aws_vpc" configuration'

TL;DR: Two resource blocks in the same module share the same type and name. Terraform loads every *.tf file in the directory, including forgotten backup files like main.bak4.tf, so grep the whole directory for the duplicated type and name, then rename one block or delete the stale copy.

The error

Error: Duplicate resource "aws_vpc" configuration

  on main.tf line 6:
   6: resource "aws_vpc" "main" {

A aws_vpc resource named "main" was already declared at vpc.tf:1,1-39. Resource names must be unique per type in each module.

Steps

1. Search the module directory: grep -rn 'resource "aws_vpc" "main"' --include='*.tf' .. Expected: two (or more) hits across your .tf files.
2. Decide which one is canonical. Rename the other block's name (for example "main" to "secondary"), or delete the file if it is a stale copy or backup. Expected: exactly one declaration remains.
3. Check for backup files Terraform still loads: ls *.tf and look for *.bak.tf, main.bak4.tf, or editor autosaves. Move them out of the directory. Expected: no stray .tf files.
4. Run terraform validate. Expected: Success! The configuration is valid.

When this applies

• validate or plan fails naming the same resource type and name twice, with "already declared at" pointing at another file.
• You copied a file as a backup inside the same directory.

When it does NOT apply

• The resource already exists in the cloud but is not in state. That is an import case (terraform import), not a config duplication.
• Intentional merging via _override.tf files. Those are a separate mechanism and do not trigger this error.

Tool and version compatibility

• All Terraform CLI versions. The uniqueness rule for type plus name per module has never changed.

Why it happens

A resource's address is its type plus its name. Two blocks with the same address in one module is ambiguous, so Terraform rejects the configuration statically. File names are cosmetic: every *.tf file in the directory is one configuration, backups included.

Edge cases and pitfalls

• Files ending in _override.tf intentionally redeclare blocks and merge instead of erroring. Do not use them casually; they make configs hard to read.
• The same rule applies to data blocks, variable blocks, and output blocks: names must be unique per kind within the module.