dedupe environment variables in amplify env type generator

[ykethan]

Problem

Issue number, if available:
#2074

Changes

Uses Set to dedupe env variables
adds test to check for duplicate variables

Corresponding docs PR, if applicable:

Validation

Checklist

• If this PR includes a functional change to the runtime behavior of the code, I have added or updated automated test coverage for this change.
• If this PR requires a change to the Project Architecture README, I have included that update in this PR.
• If this PR requires a docs update, I have linked to that docs PR above.
• If this PR modifies E2E tests, makes changes to resource provisioning, or makes SDK calls, I have run the PR checks with the run-e2e label set.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[changeset-bot]

🦋 Changeset detected

Latest commit: 56595a5

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@aws-amplify/backend-function	Patch
@aws-amplify/backend	Patch

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