compat/maven-model-builder/src/main/java/org/apache/maven/model/validation/DefaultModelValidator.java [366:424]:
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        Map<String, Plugin> index = new HashMap<>();

        for (Plugin plugin : plugins) {
            if (plugin.getGroupId() == null
                    || (plugin.getGroupId() != null
                            && plugin.getGroupId().trim().isEmpty())) {
                addViolation(
                        problems,
                        Severity.FATAL,
                        Version.V20,
                        prefix + prefix2 + "(groupId:artifactId)",
                        null,
                        "groupId of a plugin must be defined. ",
                        plugin);
            }

            if (plugin.getArtifactId() == null
                    || (plugin.getArtifactId() != null
                            && plugin.getArtifactId().trim().isEmpty())) {
                addViolation(
                        problems,
                        Severity.FATAL,
                        Version.V20,
                        prefix + prefix2 + "(groupId:artifactId)",
                        null,
                        "artifactId of a plugin must be defined. ",
                        plugin);
            }

            // This will catch cases like <version></version> or <version/>
            if (plugin.getVersion() != null && plugin.getVersion().trim().isEmpty()) {
                addViolation(
                        problems,
                        Severity.FATAL,
                        Version.V20,
                        prefix + prefix2 + "(groupId:artifactId)",
                        null,
                        "version of a plugin must be defined. ",
                        plugin);
            }

            String key = plugin.getKey();

            Plugin existing = index.get(key);

            if (existing != null) {
                addViolation(
                        problems,
                        errOn31,
                        Version.V20,
                        prefix + prefix2 + "(groupId:artifactId)",
                        null,
                        "must be unique but found duplicate declaration of plugin " + key,
                        plugin);
            } else {
                index.put(key, plugin);
            }

            Set<String> executionIds = new HashSet<>();
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impl/maven-impl/src/main/java/org/apache/maven/impl/model/DefaultModelValidator.java [694:752]:
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        Map<String, Plugin> index = new HashMap<>();

        for (Plugin plugin : plugins) {
            if (plugin.getGroupId() == null
                    || (plugin.getGroupId() != null
                            && plugin.getGroupId().trim().isEmpty())) {
                addViolation(
                        problems,
                        Severity.FATAL,
                        Version.V20,
                        prefix + prefix2 + "(groupId:artifactId)",
                        null,
                        "groupId of a plugin must be defined. ",
                        plugin);
            }

            if (plugin.getArtifactId() == null
                    || (plugin.getArtifactId() != null
                            && plugin.getArtifactId().trim().isEmpty())) {
                addViolation(
                        problems,
                        Severity.FATAL,
                        Version.V20,
                        prefix + prefix2 + "(groupId:artifactId)",
                        null,
                        "artifactId of a plugin must be defined. ",
                        plugin);
            }

            // This will catch cases like <version></version> or <version/>
            if (plugin.getVersion() != null && plugin.getVersion().trim().isEmpty()) {
                addViolation(
                        problems,
                        Severity.FATAL,
                        Version.V20,
                        prefix + prefix2 + "(groupId:artifactId)",
                        null,
                        "version of a plugin must be defined. ",
                        plugin);
            }

            String key = plugin.getKey();

            Plugin existing = index.get(key);

            if (existing != null) {
                addViolation(
                        problems,
                        errOn31,
                        Version.V20,
                        prefix + prefix2 + "(groupId:artifactId)",
                        null,
                        "must be unique but found duplicate declaration of plugin " + key,
                        plugin);
            } else {
                index.put(key, plugin);
            }

            Set<String> executionIds = new HashSet<>();
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