Prevent duplicate case generation in generate_workflows.sh

[DavidHuber-NOAA]

Description

This prevents generate_workflows.sh from generating multiple, identical test cases.

Resolves #3213

Type of change

• Bug fix (fixes something broken)

Change characteristics

• Is this a breaking change (a change in existing functionality)? YES (no longer lets you run -G and/or -E with -y)
• Does this change require a documentation update? NO
• Does this change require an update to any of the following submodules? NO

How has this been tested?

Generated test cases on Hera

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have documented my code, including function, input, and output descriptions
• My changes generate no new warnings
• This change is covered by an existing CI test or a new one has been added