Make sure the result from QCL.getElementsWithResource(name) does not include duplicates

independent-projects/bootstrap/core/src/main/java/io/quarkus/bootstrap/app/CuratedApplication.java

@@ -356,9 +356,7 @@ public QuarkusClassLoader createDeploymentClassLoader() {
             if (configuredClassLoading.isRemovedArtifact(dependency.getKey())) {
                 continue;
             }
-            if (dependency.isRuntimeCp() && dependency.isJar() &&
-                    (dependency.isReloadable() && appModel.getReloadableWorkspaceDependencies().contains(dependency.getKey()) ||
-                            configuredClassLoading.isReloadableArtifact(dependency.getKey()))) {
+            if (isReloadableRuntimeDependency(dependency)) {
                 processCpElement(dependency, element -> addCpElement(builder, dependency, element));
             }
         }
@@ -368,6 +366,12 @@ public QuarkusClassLoader createDeploymentClassLoader() {
         return builder.build();
     }
 
+    private boolean isReloadableRuntimeDependency(ResolvedDependency dependency) {
+        return dependency.isRuntimeCp() && dependency.isJar() &&
+                (dependency.isReloadable() && appModel.getReloadableWorkspaceDependencies().contains(dependency.getKey()) ||
+                        configuredClassLoading.isReloadableArtifact(dependency.getKey()));
+    }
+
     public String getClassLoaderNameSuffix() {
         return quarkusBootstrap.getBaseName() != null ? " for " + quarkusBootstrap.getBaseName() : "";
     }
@@ -405,9 +409,7 @@ public QuarkusClassLoader createRuntimeClassLoader(ClassLoader base, Map<String,
             if (configuredClassLoading.isRemovedArtifact(dependency.getKey())) {
                 continue;
             }
-            if (dependency.isRuntimeCp() && dependency.isJar() &&
-                    (dependency.isReloadable() && appModel.getReloadableWorkspaceDependencies().contains(dependency.getKey()) ||
-                            configuredClassLoading.isReloadableArtifact(dependency.getKey()))) {
+            if (isReloadableRuntimeDependency(dependency)) {
                 processCpElement(dependency, element -> addCpElement(builder, dependency, element));
             }
         }

independent-projects/bootstrap/core/src/main/java/io/quarkus/bootstrap/classloading/QuarkusClassLoader.java

@@ -625,22 +625,77 @@ public List<ClassPathElement> getElementsWithResource(String name) {
     public List<ClassPathElement> getElementsWithResource(String name, boolean localOnly) {
         ensureOpen(name);
 
-        boolean parentFirst = parentFirst(name, getClassPathResourceIndex());
+        final boolean parentFirst = parentFirst(name, getClassPathResourceIndex());
 
-        List<ClassPathElement> ret = new ArrayList<>();
+        List<ClassPathElement> result = List.of();
 
-        if (parentFirst && !localOnly && parent instanceof QuarkusClassLoader) {
-            ret.addAll(((QuarkusClassLoader) parent).getElementsWithResource(name));
+        if (parentFirst && !localOnly && parent instanceof QuarkusClassLoader parentQcl) {
+            result = parentQcl.getElementsWithResource(name);
         }
 
-        List<ClassPathElement> classPathElements = getClassPathResourceIndex().getClassPathElements(name);
-        ret.addAll(classPathElements);
+        result = joinAndDedupe(result, getClassPathResourceIndex().getClassPathElements(name));
 
-        if (!parentFirst && !localOnly && parent instanceof QuarkusClassLoader) {
-            ret.addAll(((QuarkusClassLoader) parent).getElementsWithResource(name));
+        if (!parentFirst && !localOnly && parent instanceof QuarkusClassLoader parentQcl) {
+            result = joinAndDedupe(result, parentQcl.getElementsWithResource(name));
         }
 
-        return ret;
+        return result;
+    }
+
+    /**
+     * Returns a list containing elements from two lists eliminating duplicates. Elements from the first list
+     * will appear in the result before elements from the second list.
+     * <p>
+     * The current implementation assumes that none of the lists contains duplicates on their own but some elements
+     * may be present in both lists.
+     *
+     * @param list1 first list
+     * @param list2 second list
+     * @return resulting list
+     */
+    private static <T> List<T> joinAndDedupe(List<T> list1, List<T> list2) {
+        // it appears, in the vast majority of cases at least one of the lists will be empty
+        if (list1.isEmpty()) {
+            return list2;
+        }
+        if (list2.isEmpty()) {
+            return list1;
+        }
+        final List<T> result = new ArrayList<>(list1.size() + list2.size());
+        // it looks like in most cases at this point list1 (representing elements from the parent cl) will contain only one element
+        if (list1.size() == 1) {
+            final T firstCpe = list1.get(0);
+            result.add(firstCpe);
+            for (var cpe : list2) {
+                if (cpe != firstCpe) {
+                    result.add(cpe);
+                }
+            }
+            return result;
+        }
+        result.addAll(list1);
+        for (var cpe : list2) {
+            if (!containsReference(list1, cpe)) {
+                result.add(cpe);
+            }
+        }
+        return result;
+    }
+
+    /**
+     * Checks whether a list contains an element that references the other argument.
+     *
+     * @param list list of elements
+     * @param e element to look for
+     * @return true if the list contains an element referencing {@code e}, otherwise - false
+     */
+    private static <T> boolean containsReference(List<T> list, T e) {
+        for (int i = list.size() - 1; i >= 0; --i) {
+            if (e == list.get(i)) {
+                return true;
+            }
+        }
+        return false;
     }
 
     public Set<String> getReloadableClassNames() {
@@ -902,6 +957,10 @@ public QuarkusClassLoader build() {
             return new QuarkusClassLoader(this);
         }
 
+        @Override
+        public String toString() {
+            return "QuarkusClassLoader.Builder:" + name + "@" + Integer.toHexString(hashCode());
+        }
     }
 
     public ClassLoader parent() {