Explanation
validate will look for syntax errors "Declaring a resource identifier more than once" is a syntax error

https://www.terraform.io/docs/commands/environment-variables.html

Explanation
Infrastructure as Code is not used to develop applications, but it can be used to help deploy or provision those
applications to a public cloud provider or on-premises infrastructure.
All of the others are benefits to using Infrastructure as Code over the traditional way of managing
infrastructure, regardless if it's public cloud or on-premises.

Explanation
For larger infrastructures, querying every resource is too slow. Many cloud providers do not provide APIs to
query multiple resources at once, and the round trip time for each resource is hundreds of milliseconds. On top
of this, cloud providers almost always have API rate limiting so Terraform can only request a certain number
of resources in a period of time. Larger users of Terraform make heavy use of the -refresh=false flag as well as
the -target flag in order to work around this. In these scenarios, the cached state is treated as the record of
truth.
Although 'Use -refresh=false flag as well as the -target flag with terraform plan in order to work around this.' is
a solution, but its not always recommended. Instead of using -target as a means to operate on isolated portions
of very large configurations, prefer instead to break large configurations into several smaller configurations
that can each be independently applied. Data sources can be used to access information about resources
created in other configurations, allowing a complex system architecture to be broken down into more
manageable parts that can be updated independently.
Option 'Run terraform refresh every time before running terraform plan.' and 'Use -refresh=true flag as well as
the -target flag with terraform plan in order to work around this.' is not correct because in both the cases
terraform will query every resources of the infrastructure.

Explanation
To upgrade to the latest acceptable version of each provider, run terraform init -upgrade. This command also upgrades to the latest versions of all Terraform modules.
https://www.terraform.io/docs/configuration/providers.html