Terraform: how to not duplicate security groups when creating it using modules?

Update

I don't get it, but I reran terraform apply and it did not try to duplicate resources (no errors). Now it checks resources correctly. Kind of unexpected end of events.

I'm learning Terraform, and I created module to allow creating some basic security groups. It runs fine first time and creates resources as expected. But if I run terraform apply second time, it tries to create same groups again and then I get duplicate error, because such security groups already exist.

If I would create security groups directly without module, Terraform recognizes it and does not try to recreate existing resources.

I probably do something wrong here.

Here is my module and how I try to use it:

My project structure looks like this

├── main.tf
├── modules
│   ├── security_group_ec2
│   │   ├── main.tf
│   │   ├── outputs.tf
│   │   └── variables.tf
│   └── security_group_rds
│       ├── main.tf
│       ├── outputs.tf
│       └── variables.tf
├── scripts
│   └── update-odoo-cfg.py
├── security_groups.tf
├── terraform.tfstate
├── terraform.tfstate.backup
├── variables.tf
└── vpc.tf

Now my security_group_ec2 content:

main.tf:

resource "aws_security_group" "sg" {
  name = "${var.name}"
  description = "${var.description}"
  vpc_id = "${var.vpc_id}"

  ingress {
    from_port   = 80
    to_port     = 80
    protocol    = "tcp"
    cidr_blocks = ["0.0.0.0/0"]
  }

  ingress {
    from_port   = 443
    to_port     = 443
    protocol    = "tcp"
    cidr_blocks = ["0.0.0.0/0"]
  }

  ingress {
    from_port   = 22
    to_port     = 22
    protocol    = "tcp"
    cidr_blocks = ["0.0.0.0/0"]
  }

  egress {
    from_port       = 0
    to_port         = 0
    protocol        = "-1"
    cidr_blocks     = ["0.0.0.0/0"]
  }
}

variables.tf:

variable "name" {
  description = "Name of security group"
}

variable "description" {
  description = "Description of security group"
}
variable "vpc_id" {
  description = "Virtual Private Cloud ID to assign"
}

outputs:

output "sg_id" {
  value = "${aws_security_group.sg.id}"
}

And this is file were I call module to create two security groups.

security_groups.tf:

# EC2
module "security_group_staging_ec2" {
  source = "modules/security_group_ec2"
  name = "ec2_staging_sg"
  description = "EC2 Staging Security Group"
  vpc_id = "${aws_default_vpc.default.id}"
}

module "security_group_prod_ec2" {
  source = "modules/security_group_ec2"
  name = "ec2_prod_sg"
  description = "EC2 Production Security Group"
  vpc_id = "${aws_default_vpc.default.id}"
}

This is error output when running terraform apply:

module.security_group_staging.aws_security_group.sg: Destruction complete after 1s
module.security_group_prod.aws_security_group.sg: Destruction complete after 1s

Error: Error applying plan:

2 error(s) occurred:

* module.security_group_staging_ec2.aws_security_group.sg: 1 error(s) occurred:

* aws_security_group.sg: Error creating Security Group: InvalidGroup.Duplicate: The security group 'ec2_staging_sg' already exists for VPC 'vpc-2a84a741'
    status code: 400, request id: 835004f0-d8a1-4ed5-8e21-17f01eb18a23
* module.security_group_prod_ec2.aws_security_group.sg: 1 error(s) occurred:

* aws_security_group.sg: Error creating Security Group: InvalidGroup.Duplicate: The security group 'ec2_prod_sg' already exists for VPC 'vpc-2a84a741'
    status code: 400, request id: 953b23e8-20cb-4ccb-940a-6a9ddab54d53

Terraform does not automatically rollback in the face of errors.
Instead, your Terraform state file has been partially updated with
any resources that successfully completed. Please address the error
above and apply again to incrementally change your infrastructure.

P.S. I probably need to somehow indicate that resource is created when calling module?