What happens if running "terraform apply" twice? Does it create all the resources twice?
I'm assuming that when you say "terraform deploy" here you mean running the terraform apply command.
The first time you run terraform apply against an entirely new configuration, Terraform will propose to create new objects corresponding with each of the resource instances you declared in the configuration. If you accept the plan and thus allow Terraform to really apply it, Terraform will create each of those objects and record information about them in the Terraform state.
If you then run terraform apply again, Terraform will compare your configuration with the state to see if there are any differences. This time, Terraform will propose changes only if the configuration doesn't match the existing objects that are recorded in the state. If you accept that plan then Terraform will take each of the actions it proposed, which can be a mixture of different action types: update, create, destroy.
This means that in order to use Terraform successfully you need to make sure to keep the state snapshots safe between Terraform runs. With no special configuration at all Terraform will by default save the state in a local file called terraform.tfstate, but when you are using Terraform in production you'll typically use remote state, which is a way to tell Terraform to store state snapshots in a remote data store separate from the computer where you are running Terraform. By storing the state in a location that all of your coworkers can access, you can collaborate together.
If you use Terraform Cloud, a complementary hosted service provided by HashiCorp, you can configure Terraform to store the state snapshots in Terraform Cloud itself. Terraform Cloud has various other capabilities too, such as running Terraform in a remote execution environment so that everyone who uses that environment can be sure to run Terraform with a consistent set of environment variables stored remotely.