Steps to reproduce:
1. Create new Unity project
2. Open Package Manager
3. Create a new Package
4. Select to edit new Package Manifest in Inspector
5. In Inspector, Dependencies press “+” to add multiple packages with versions
6. Press “Apply”
7. Observe the Console window

Actual results: Error is thrown: “ArgumentException: An item with the same key has already been added. Key: com.unity.test-framework
System.Collections.Generic.Dictionary`2[TKey,TValue].TryInsert (TKey key, TValue value, System.Collections.Generic.InsertionBehavior behavior) (at <c818795a2e354f2faac5f8cee1240e70>:0)
System.Collections.Generic.Dictionary`2[TKey,TValue].Add (TKey key, TValue value) (at <c818795a2e354f2faac5f8cee1240e70>:0)
UnityEditor.PackageManager.UI.Internal.PackageManifestImporterEditor.WritePackageManifest (UnityEngine.Object target, UnityEditor.PackageManager.UI.Internal.PackageManifestImporterEditor+PackageManifestState packageState) (at /Users/bokken/build/output/unity/unity/Modules/PackageManagerUI/Editor/UI/PackageManifestImporterEditor.cs:829)”

Expected results: Adding a package with the same name and version should not be allowed, “Apply” button should be greyed out

Reproducible with versions: 6000.3.5f1, 6000.4.0b5, 6000.5.0a5
Can’t test with versions: 6000.0.66f1 (no such functionality)

Tested on (OS): macOS Silicon Tahoe 26.2 (M1)