AddToTagVisitor: opt-in flag to skip insert when a semantically-equal child exists

Raw XML allows duplicate sibling tags, so "no semantic duplicates" is a
schema-specific concern (Maven POM, csproj, etc.) rather than something
that should be baked into the schema-agnostic rewrite-xml primitive.

This change makes the SemanticallyEqual guard opt-in via a new
`skipIfEquivalentExists` boolean (default `false`). Callers wanting
idempotent inserts pass `true`; existing call sites keep their current
duplicate-permitting behavior unchanged.

Two new constructor overloads are added so callers don't have to pass
`null` for `tagComparator` just to opt in:
- `(Xml.Tag, Xml.Tag, boolean)`
- `(Xml.Tag, Xml.Tag, @nullable Comparator<Content>, boolean)`

Author: steve-aom-elliott

rewrite-xml/src/main/java/org/openrewrite/xml/AddToTagVisitor.java

@@ -33,19 +33,37 @@ public class AddToTagVisitor<P> extends XmlVisitor<P> {
     @Nullable
     private final Comparator<Content> tagComparator;
 
+    private final boolean skipIfEquivalentExists;
+
     public AddToTagVisitor(Xml.Tag scope, Xml.Tag tagToAdd) {
-        this(scope, tagToAdd, null);
+        this(scope, tagToAdd, null, false);
+    }
+
+    public AddToTagVisitor(Xml.Tag scope, Xml.Tag tagToAdd, boolean skipIfEquivalentExists) {
+        this(scope, tagToAdd, null, skipIfEquivalentExists);
     }
 
     public AddToTagVisitor(Xml.Tag scope, Xml.Tag tagToAdd, @Nullable Comparator<Content> tagComparator) {
+        this(scope, tagToAdd, tagComparator, false);
+    }
+
+    public AddToTagVisitor(Xml.Tag scope, Xml.Tag tagToAdd, @Nullable Comparator<Content> tagComparator, boolean skipIfEquivalentExists) {
         this.scope = scope;
         this.tagToAdd = tagToAdd;
         this.tagComparator = tagComparator;
+        this.skipIfEquivalentExists = skipIfEquivalentExists;
     }
 
     @Override
     public Xml visitTag(Xml.Tag t, P p) {
         if (scope.isScope(t)) {
+            if (skipIfEquivalentExists && t.getContent() != null) {
+                for (Content existing : t.getContent()) {
+                    if (existing instanceof Xml.Tag && SemanticallyEqual.areEqual(existing, tagToAdd)) {
+                        return super.visitTag(t, p);
+                    }
+                }
+            }
             assert getCursor().getParent() != null;
             if (t.getClosing() == null) {
                 t = t.withClosing(autoFormat(new Xml.Tag.Closing(Tree.randomId(), "\n",

rewrite-xml/src/test/java/org/openrewrite/xml/AddToTagTest.java

@@ -143,6 +143,73 @@ public Xml visitDocument(Xml.Document x, ExecutionContext ctx) {
         );
     }
 
+    @Test
+    void doesNotAddSemanticallyEqualDuplicate() {
+        rewriteRun(
+          spec -> spec.recipe(toRecipe(() -> new XmlVisitor<>() {
+              @Override
+              public Xml visitDocument(Xml.Document x, ExecutionContext ctx) {
+                  doAfterVisit(new AddToTagVisitor<>(x.getRoot(), Xml.Tag.build("<bean id=\"myBean\"/>"), true));
+                  return super.visitDocument(x, ctx);
+              }
+          })),
+          xml(
+            """
+              <beans>
+                <bean id="myBean"/>
+              </beans>
+              """
+          )
+        );
+    }
+
+    @Test
+    void doesNotAddDuplicateIgnoringAttributeOrder() {
+        rewriteRun(
+          spec -> spec.recipe(toRecipe(() -> new XmlVisitor<>() {
+              @Override
+              public Xml visitDocument(Xml.Document x, ExecutionContext ctx) {
+                  doAfterVisit(new AddToTagVisitor<>(x.getRoot(),
+                          Xml.Tag.build("<bean class=\"C\" id=\"myBean\"/>"), true));
+                  return super.visitDocument(x, ctx);
+              }
+          })),
+          xml(
+            """
+              <beans>
+                <bean id="myBean" class="C"/>
+              </beans>
+              """
+          )
+        );
+    }
+
+    @Test
+    void addsWhenChildrenShareNameButDifferentAttributes() {
+        rewriteRun(
+          spec -> spec.recipe(toRecipe(() -> new XmlVisitor<>() {
+              @Override
+              public Xml visitDocument(Xml.Document x, ExecutionContext ctx) {
+                  doAfterVisit(new AddToTagVisitor<>(x.getRoot(), Xml.Tag.build("<bean id=\"myBean2\"/>"), true));
+                  return super.visitDocument(x, ctx);
+              }
+          })),
+          xml(
+            """
+              <beans>
+                <bean id="myBean"/>
+              </beans>
+              """,
+            """
+              <beans>
+                <bean id="myBean"/>
+                <bean id="myBean2"/>
+              </beans>
+              """
+          )
+        );
+    }
+
     @Issue("https://github.com/openrewrite/rewrite/issues/1392")
     @Test
     void preserveNonTagContent() {