Avoid double-marking variables

[pselle]

It is possible, say with multiple layers of sensitive variables, to "double-mark" a variable. Add a check to ensure this does not happen.

Fixes #27095

[codecov]

Codecov Report

Merging #27131 (cdbd68e) into master (6bacc7a) will increase coverage by 54.69%.
The diff coverage is 100.00%.

Impacted Files	Coverage Δ
terraform/evaluate.go	52.89% <100.00%> (ø)
internal/getproviders/hash.go	42.05% <0.00%> (ø)
terraform/node_provider_abstract.go	66.66% <0.00%> (ø)
states/statemgr/helper.go	0.00% <0.00%> (ø)
terraform/node_resource_plan_destroy.go	80.00% <0.00%> (ø)
configs/resource.go	80.46% <0.00%> (ø)
command/013_config_upgrade.go	38.46% <0.00%> (ø)
lang/blocktoattr/fixup.go	74.57% <0.00%> (ø)
states/statemgr/statemgr_fake.go	54.54% <0.00%> (ø)
terraform/node_resource_abstract_instance.go	81.94% <0.00%> (ø)
... and 537 more

[alisdair]

Looks good to me! I appreciate the two different levels of test coverage added.

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

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