Raise when duplicate GEN_KW keys

[andreas-el]

Issue
Resolves #6074

Approach
Verifies that GEN_KW keys are unique

Pre review checklist

• Read through the code changes carefully after finishing work
• Make sure tests pass locally (after every commit!)
• Prepare changes in small commits for more convenient review (optional)
• PR title captures the intent of the changes, and is fitting for release notes.
• Updated documentation
• Ensured that unit tests are added for all new behavior (See
  Ground Rules),
  and changes to existing code have good test coverage.

Pre merge checklist

• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.

[codecov-commenter]

Codecov Report

Merging #6109 (b9007a8) into main (d1ae20d) will increase coverage by 0.00%.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #6109   +/-   ##
=======================================
  Coverage   82.48%   82.49%           
=======================================
  Files         350      350           
  Lines       21486    21491    +5     
  Branches      826      826           
=======================================
+ Hits        17723    17728    +5     
  Misses       3469     3469           
  Partials      294      294           

Files Changed	Coverage Δ
src/ert/config/gen_kw_config.py	98.07% <100.00%> (+0.03%)	⬆️

... and 4 files with indirect coverage changes

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[DanSava]

LGTM 🍍