apache/maven · error · XMLStreamException

Duplicated tag: '{}'

Error message

Duplicated tag: '{}'

What it means

checkDuplicate in the classic reader.vm template, flats branch: recognized child tags that are not flat mappings reach the switch default, where parsed.add(tagName) returning false (tag already seen under this parent) throws. It enforces single-occurrence fields regardless of strict mode, in readers generated without the newer template's extras.

Source

Thrown at src/mdo/reader.vm:634

#end
#if ( ! ${aliases.isEmpty()} )
        switch (tagName) {
  #foreach( $entry in $aliases.entrySet() )
            case "${entry.key}":
                tagName = "${entry.value}";
                break;
  #end
        }
#end
#if ( ! ${flats.isEmpty()} )
        switch (tagName) {
  #foreach( $entry in $flats.entrySet() )
            case "${entry.key}":
  #end
                break;
            default:
                if (!parsed.add(tagName)) {
                    throw new XMLStreamException("Duplicated tag: '" + tagName + "'", parser.getLocation(), null);
                }
        }
#else
        if (!parsed.add(tagName)) {
            throw new XMLStreamException("Duplicated tag: '" + tagName + "'", parser.getLocation(), null);
        }
#end
        return tagName;
    }

    /**
     * Sets the state of the "add default entities" flag.
     *
     * @param addDefaultEntities a addDefaultEntities object.
     */
    public void setAddDefaultEntities(boolean addDefaultEntities) {
        this.addDefaultEntities = addDefaultEntities;
    } //-- void setAddDefaultEntities(boolean)

View on GitHub (pinned to e4093d4e12)

Solutions

  1. Remove the duplicate element; the exception location identifies the second occurrence.
  2. Schema-validate first for clearer maxOccurs diagnostics.
  3. Model-level fix (list field) required if repetition is legitimate.

Example fix

<!-- before -->
<project xmlns="http://maven.apache.org/POM/4.0.0">
  <packaging>jar</packaging>
  <packaging>war</packaging>
</project>

<!-- after -->
<project xmlns="http://maven.apache.org/POM/4.0.0">
  <packaging>war</packaging>
</project>
Defensive patterns

Strategy: try-catch

Validate before calling

// XSD validation catches duplicate singular elements before the model read
SchemaFactory sf = SchemaFactory.newInstance(XMLConstants.W3C_XML_SCHEMA_NS_URI);
sf.newSchema(new StreamSource(xsdFile)).newValidator().validate(new StreamSource(xmlFile));

Try / catch

try {
    Model model = reader.read(in, true);
} catch (XMLStreamException e) {
    if (e.getMessage() != null && e.getMessage().startsWith("Duplicated tag")) {
        // report e.getLocation() with the file name
    }
    throw e;
}

Prevention

When it happens

Trigger: A singular child element repeated inside its parent: two <packaging> under <project>, two <systemPath> in one dependency, two <name> in the same element - thrown in strict and lenient reads alike.

Common situations: Copy-paste editing; merges keeping both blocks; generators appending a field twice.

Related errors

AI-assisted analysis of apache/maven@e4093d4e12 (2026-08-21). Data as JSON: /api/errors/9a21afdc908b92fe. Report an issue: GitHub.