Fix bug where duplicate dates in summary caused update to crash

[oyvindeide]

We drop all duplicate time steps from the summary dates, and only read the first one.

Issue
Resolves #5967

Approach
Short description of the approach

Pre review checklist

• Read through the code changes carefully after finishing work
• Make sure tests pass locally (after every commit!)
• Wait for tests in CI to pass (see "checks" below)
• 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 new behaviour is tested (opened GUI? screenshots? tried workflows?)
• Commit history is consistent and clean, in line with the contribution guidelines.

Pre merge checklist

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

[dafeda]

Seems reasonable 👍

[codecov-commenter]

Codecov Report

Merging #6014 (a7c48fb) into main (61ec478) will decrease coverage by 0.05%.
Report is 5 commits behind head on main.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##             main    #6014      +/-   ##
==========================================
- Coverage   82.49%   82.44%   -0.05%     
==========================================
  Files         350      350              
  Lines       21486    21444      -42     
  Branches      826      837      +11     
==========================================
- Hits        17725    17680      -45     
+ Misses       3467     3466       -1     
- Partials      294      298       +4     

Files Changed	Coverage Δ
src/clib/lib/enkf/read_summary.cpp	0.00% <0.00%> (ø)
src/ert/config/summary_config.py	100.00% <100.00%> (ø)

... and 14 files with indirect coverage changes

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[eivindjahren]

👍