apache/maven · error · DuplicateProjectException

Two or more projects in the reactor have the same identifier

Error message

Two or more projects in the reactor have the same identifier, please make sure that <groupId>:<artifactId>:<version> is unique for each project: {}

What it means

While collecting the reactor project index, DefaultMaven groups projects by groupId:artifactId:version and throws DuplicateProjectException when two or more reactor projects share the same GAV; the message prints the collisions map (GAV -> list of offending pom files). Reactor identity must be unique because dependency resolution inside the session keys on it.

Source

Thrown at impl/maven-core/src/main/java/org/apache/maven/DefaultMaven.java:633

            MavenProject collision = index.get(projectId);

            if (collision == null) {
                index.put(projectId, project);
            } else {
                List<File> pomFiles = collisions.get(projectId);

                if (pomFiles == null) {
                    pomFiles = new ArrayList<>(Arrays.asList(collision.getFile(), project.getFile()));
                    collisions.put(projectId, pomFiles);
                } else {
                    pomFiles.add(project.getFile());
                }
            }
        }

        if (!collisions.isEmpty()) {
            throw new DuplicateProjectException(
                    "Two or more projects in the reactor"
                            + " have the same identifier, please make sure that <groupId>:<artifactId>:<version>"
                            + " is unique for each project: " + collisions,
                    collisions);
        }

        return index;
    }

    private Result<? extends ProjectDependencyGraph> buildGraph(MavenSession session) {
        Result<? extends ProjectDependencyGraph> graphResult = graphBuilder.build(session);
        for (ModelProblem problem : graphResult.getProblems()) {
            if (problem.getSeverity() == ModelProblem.Severity.WARNING) {
                logger.warn(problem.getMessage());
            } else {
                logger.error(problem.getMessage());
            }
        }

View on GitHub (pinned to e4093d4e12)

Solutions

Read the message: it lists each duplicated GAV and every pom file involved - open those poms.
Make each module's artifactId (or version) unique, or remove the duplicate/overlapping `<module>` entry.
Check `<modules>` in all profiles for the same path twice; normalize relative paths (drop `./`).
Ensure child modules declare their own artifactId instead of inheriting the parent's.
Run `mvn validate` after restructuring to confirm the reactor is unique.

Example fix

<!-- before: same module reachable twice -->
<modules>
  <module>app</module>
  <module>./app</module>
</modules>

<!-- after -->
<modules>
  <module>app</module>
</modules>

Defensive patterns

Strategy: try-catch

Try / catch

try {
    MavenExecutionResult result = maven.execute(request);
    for (Throwable t : result.getExceptions()) {
        if (t instanceof DuplicateProjectException dpe) {
            // dpe.getCollisions() maps GAV -> offending pom files
            reportDuplicateProjects(dpe.getCollisions());
        }
    }
} catch (DuplicateProjectException dpe) {
    reportDuplicateProjects(dpe.getCollisions());
}

Prevention

Lint aggregator POMs for duplicate module paths and duplicate child GAVs in CI.
Always set an explicit artifactId in child modules.
Normalize module paths (no `./` prefixes) to avoid accidental double inclusion.

When it happens

Trigger: The same module reachable twice (e.g. `<module>app</module>` and `<module>./app</module>`); overlapping module paths across profiles; two submodules copy-pasted without changing artifactId; a child POM that fails to override the parent's artifactId.

Common situations: Copy-pasted module scaffolds; profile combinations only activated together in CI; restructured repos with stale `<module>` entries; aggregator POMs including a directory both directly and via a wildcard-ish path.

Related errors

AI-assisted analysis of apache/maven@e4093d4e12 (2026-08-21). Data as JSON: /api/errors/9abd4c01097e4903. Report an issue: GitHub.