Update classpath settings: avoid duplicate build path for libs

When computing the classpath entry for a library found in the PDE
plugin, the result has export=true and no source attachment.
If the already existing classpath entry differs in whatever irrelevant
parameter, the computed classpath entry is added as well, so Update
classpath settings fails with "Build path contains duplicate entry".

Instead, use the library path only to check for the already existing
classpath entry.

ui/org.eclipse.pde.core/src/org/eclipse/pde/internal/core/ClasspathComputer.java

@@ -124,7 +124,7 @@ private static void addSourceAndLibraries(IProject project, IPluginModelBase mod
 				if (library.getName().equals(".")) { //$NON-NLS-1$
 					addJARdPlugin(project, ClasspathUtilCore.getFilename(model), sourceAttachment, attrs, result);
 				} else {
-					addLibraryEntry(project, library, sourceAttachment, attrs, result);
+					addLibraryEntry(project, library, sourceAttachment, attrs, paths, result);
 				}
 			}
 		}
@@ -236,7 +236,7 @@ protected static IBuild getBuild(IProject project) throws CoreException {
 		return (buildModel != null) ? buildModel.getBuild() : null;
 	}
 
-	private static void addLibraryEntry(IProject project, IPluginLibrary library, IPath sourceAttachment, IClasspathAttribute[] attrs, ArrayList<IClasspathEntry> result) throws JavaModelException {
+	private static void addLibraryEntry(IProject project, IPluginLibrary library, IPath sourceAttachment, IClasspathAttribute[] attrs, HashSet<IPath> paths, ArrayList<IClasspathEntry> result) throws JavaModelException {
 		String name = ClasspathUtilCore.expandLibraryName(library.getName());
 		IResource jarFile = project.findMember(name);
 		if (jarFile == null) {
@@ -248,15 +248,15 @@ private static void addLibraryEntry(IProject project, IPluginLibrary library, IP
 			IClasspathEntry oldEntry = root.getRawClasspathEntry();
 			// If we have the same binary root but new or different source, we should recreate the entry
 			if ((sourceAttachment == null && oldEntry.getSourceAttachmentPath() != null) || (sourceAttachment != null && sourceAttachment.equals(oldEntry.getSourceAttachmentPath()))) {
-				if (!result.contains(oldEntry)) {
+				if (paths.add(oldEntry.getPath())) {
 					result.add(oldEntry);
 					return;
 				}
 			}
 		}
 
 		IClasspathEntry entry = createClasspathEntry(project, jarFile, name, sourceAttachment, attrs, library.isExported());
-		if (!result.contains(entry)) {
+		if (paths.add(entry.getPath())) {
 			result.add(entry);
 		}
 	}