Fix DuplicateColumn bug in save_parameters_numpy()

[jonathan-eq]

Issue
Resolves #12706

Approach
Short description of the approach

(Screenshot of new behavior in GUI if applicable)

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'just rapid-tests')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Add backport label to latest release (format: 'backport release-branch-name')

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.63%. Comparing base (c528c39) to head (37f3120).
⚠️ Report is 21 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12707      +/-   ##
==========================================
- Coverage   90.67%   90.63%   -0.05%     
==========================================
  Files         429      431       +2     
  Lines       29803    30113     +310     
==========================================
+ Hits        27025    27292     +267     
- Misses       2778     2821      +43     

Flag	Coverage Δ
cli-tests	37.30% <100.00%> (-0.27%)	⬇️
gui-tests	68.67% <100.00%> (-0.74%)	⬇️
performance-and-unit-tests	74.06% <100.00%> (+0.13%)	⬆️
test	37.50% <0.00%> (-0.58%)	⬇️

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[codspeed-hq]

Merging this PR will not alter performance

✅ 23 untouched benchmarks

---

_{Comparing jonathan-eq:fix-bugss (37f3120) with main (fbdfd8b)}

[xjules]

Nice @jonathan-eq ! Could you maybe update the commit message bit to explain where was this issue?

[Copilot]

Pull request overview

This PR fixes a bug in save_parameters_numpy() where calling the method multiple times with the same parameter name would raise a DuplicateColumn error instead of properly overwriting existing values.

Changes:

• Added logic to drop existing columns (except "realization") before joining new parameter data
• Added a regression test verifying that multiple saves to the same parameter successfully overwrite previous values

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/ert/storage/local_ensemble.py	Drops duplicate columns before joining to prevent DuplicateColumn errors
tests/ert/unit_tests/storage/test_local_storage.py	Adds regression test for multiple parameter saves

[scout-team-app]

Successfully created backport PR for version-18.0:

• [Backport version-18.0] Fix DuplicateColumn bug in save_parameters_numpy() #12709

[scout-team-app]

Successfully created backport PR for version-19.0:

• [Backport version-19.0] Fix DuplicateColumn bug in save_parameters_numpy() #12710