UnityYAMLMerge duplicates list elements when there are changes on both sides

Reproduction steps:

1. Download and extract the “IN-139130” project folder
2. Open a new Terminal window at the “IN-139130/MergeTests“ folder
3. Run the following command: (Edit it to fit the exact scenario)
   /Applications/Unity/Hub/Editor/6000.0.67f1/Unity.app/Contents/Tools/UnityYAMLMerge merge -p baseS.unity theirsS.unity yoursS.unity resultS.unity
   Note: In 6000.3 and up, the “Tools” folder has been renamed to “Helpers”
4. Open the “IN-139130/MergeTests/resultS.unity” file with any text editor app (like Visual Studio Code)
5. Observe the mTerms property in lines 344-366

Actual results: The “mTerms” property in line 344 has two copies of the same element ID (“-1154547082”) with different values meaning it silently duplicated these elements instead of marking a conflict
Expected result: The “mTerms” has the elements merged or a conflict is marked for the merge

Reproducible with: 6000.0.72f1, 6000.3.13f1, 60004.2f1, 6000.5.0b3, 6000.6.0a2

Reproducible on: macOS 26.4 (M3 Max), macOS 15.7.5 (by reporter)
Not reproducible on: No other environments tested

Note: This is reproducible with both .unity (Scene) files and .asset files