Fixes #3169 - handle duplicate dimensions

[tclune]

Issue #3169 was incorrect - code was not trapping duplicate dimensions at all. Now throws exception if duplicate dim name has different extent.

Types of change(s)

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Trivial change (affects only documentation or cleanup)
• [ ] Refactor (no functional changes, no api changes)

Checklist

• [ ] Tested this change with a run of GEOSgcm
• [x] Ran the Unit Tests (make tests)

Description

Related Issue

[tclune]

Good enough on CI for trivial change.