🚨 OpenTofu Error: 'Duplicate resource configuration' — Cause, Fix, and Troubleshooting Guide

What this error means

A resource address — the type plus the name, like aws_instance.web — must be unique within a module. If two blocks declare the same type and name, even in different .tf files, OpenTofu cannot tell them apart and refuses to load the configuration:

Error: Duplicate resource "aws_instance" "web" configuration

  on instances.tf line 20, in resource "aws_instance" "web":
  20: resource "aws_instance" "web" {

A aws_instance resource named "web" was already declared at main.tf:8,1-33.
Resource names must be unique per type in each module.

When it appears

• tofu validate/plan fails with Duplicate resource ... configuration.
• The message names both locations: the new declaration and the earlier one.
• Common after copy-pasting a block, splitting one file into several, or merging branches.
• The same error shape appears for duplicate data, variable, output, module, and provider blocks.

Configuration causes

• Copy-paste — a block duplicated within or across files without renaming.
• File split — moving resources into new files left a copy behind in the original.
• Merge conflict resolved wrong — both sides of a merge kept the same block.
• count/for_each misuse — trying to express multiples with two identical blocks instead of one meta-argument.
• Re-declared provider or variable with the same name.

Validating the configuration

OpenTofu names both files and lines — go straight there:

tofu validate

Find every declaration of the address across the module:

grep -rn 'resource "aws_instance" "web"' *.tf

Check for duplicated variables/outputs too:

grep -rhn 'variable "' *.tf | sort | uniq -d

Resolution

Rename or delete one of the duplicates so each address is unique:

resource "aws_instance" "web"     { ... }
resource "aws_instance" "web_api" { ... }   # was a second "web"

Use count or for_each instead of two near-identical blocks:

resource "aws_instance" "web" {
  for_each      = toset(["a", "b"])
  ami           = var.ami
  instance_type = "t3.micro"
  tags          = { Name = "web-${each.key}" }
}

Resolve merge duplicates by keeping exactly one copy of the block.

Re-validate after the edit:

tofu validate && tofu plan

Keeping definitions valid

• OpenTofu treats all .tf files in a directory as one module — splitting files does not create separate namespaces.
• Multiplicity is expressed with count/for_each, never by duplicating blocks.
• The same uniqueness rule applies to data, variable, output, module, and provider (per alias) blocks.
• Run tofu validate in pre-commit so duplicates from merges are caught before review.

Related configuration errors

• OpenTofu Error: ‘Cycle’ — Cause, Fix, and Troubleshooting Guide
• OpenTofu Error: ‘Invalid count argument’ — Cause, Fix, and Troubleshooting Guide
• OpenTofu Error: ‘Reference to undeclared resource’ — Cause, Fix, and Troubleshooting Guide