Fix IllegalArgumentException with duplicate natures

Changed ProjectDescription.hasPrivateChanges() to use HashSet instead of
Set.of() to handle duplicate natures gracefully. Set.of() throws
IllegalArgumentException on duplicates, but HashSet tolerates them and
correctly compares nature sets.

Also added test case testDuplicateNatures() to ensure this regression
doesn't occur again.

Author: Copilot

resources/bundles/org.eclipse.core.resources/src/org/eclipse/core/internal/resources/ProjectDescription.java

@@ -24,6 +24,7 @@
 import java.util.Collection;
 import java.util.Collections;
 import java.util.HashMap;
+import java.util.HashSet;
 import java.util.LinkedHashSet;
 import java.util.LinkedList;
 import java.util.List;
@@ -581,7 +582,7 @@ public boolean hasPrivateChanges(ProjectDescription description) {
 			return true;
 		}
 		// has natures changed?
-		if (!Set.of(natures).equals(Set.of(description.natures))) {
+		if (!new HashSet<>(Arrays.asList(natures)).equals(new HashSet<>(Arrays.asList(description.natures)))) {
 			return true;
 		}
 		// has buildspec changed?

resources/tests/org.eclipse.core.tests.resources/src/org/eclipse/core/tests/resources/NatureTest.java

@@ -167,6 +167,30 @@ public void testInvalidRemovals() throws Throwable {
 		assertHasEnabledNature(NATURE_SNOW);
 	}
 
+	/**
+	 * Tests that duplicate natures in the project description don't cause
+	 * IllegalArgumentException. This is a regression test for a bug where
+	 * Set.of() was used which throws on duplicates.
+	 */
+	@Test
+	public void testDuplicateNatures() throws Throwable {
+		createInWorkspace(project);
+
+		// Set initial nature
+		setNatures(project, new String[] { NATURE_SIMPLE }, false);
+		assertHasEnabledNature(NATURE_SIMPLE);
+
+		// Try to set natures with a duplicate - this should not throw IllegalArgumentException
+		// The duplicate should be handled gracefully (deduplication happens automatically)
+		IProjectDescription desc = project.getDescription();
+		desc.setNatureIds(new String[] { NATURE_SIMPLE, NATURE_SIMPLE });
+		// This should not throw IllegalArgumentException when hasPrivateChanges is called
+		project.setDescription(desc, IResource.KEEP_HISTORY, createTestMonitor());
+		
+		// After deduplication, only one instance of the nature should remain
+		assertHasEnabledNature(NATURE_SIMPLE);
+	}
+
 	@Test
 	public void testNatureLifecyle() throws Throwable {
 		createInWorkspace(project);