diff --git a/smack-core/src/test/java/org/jivesoftware/smack/util/XmlStringBuilderTest.java b/smack-core/src/test/java/org/jivesoftware/smack/util/XmlStringBuilderTest.java
new file mode 100644
index 000000000..8436a4dd2
--- /dev/null
+++ b/smack-core/src/test/java/org/jivesoftware/smack/util/XmlStringBuilderTest.java
@@ -0,0 +1,47 @@
+/**
+ *
+ * Copyright 2019 Florian Schmaus.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.jivesoftware.smack.util;
+
+import org.jivesoftware.smack.packet.StandardExtensionElement;
+import org.jivesoftware.smack.packet.XmlEnvironment;
+import org.jivesoftware.smack.test.util.XmlUnitUtils;
+
+import org.junit.jupiter.api.Test;
+
+public class XmlStringBuilderTest {
+
+    /**
+     * Test that {@link XmlStringBuilder} does not omit the second inner namespace declaration.
+     */
+    @Test
+    public void equalInnerNamespaceTest() {
+        StandardExtensionElement innerOne = StandardExtensionElement.builder("inner", "inner-namespace").build();
+        StandardExtensionElement innerTwo = StandardExtensionElement.builder("inner", "inner-namespace").build();
+
+        StandardExtensionElement outer = StandardExtensionElement.builder("outer", "outer-namespace").addElement(
+                        innerOne).addElement(innerTwo).build();
+
+        String expectedXml = "<outer xmlns='outer-namespace'><inner xmlns='inner-namespace'></inner><inner xmlns='inner-namespace'></inner></outer>";
+        XmlStringBuilder actualXml = outer.toXML(XmlEnvironment.EMPTY);
+
+        XmlUnitUtils.assertXmlSimilar(expectedXml, actualXml);
+
+        StringBuilder actualXmlTwo = actualXml.toXML(XmlEnvironment.EMPTY);
+
+        XmlUnitUtils.assertXmlSimilar(expectedXml, actualXmlTwo);
+    }
+}