* referencing the type variables seem worthwhile.
   */
  final void checkNoDuplicate(R rowKey, C columnKey, @Nullable V existingValue, V newValue) {
    checkArgument(
        existingValue == null,
        "Duplicate key: (row=%s, column=%s), values: [%s, %s].",
        rowKey,
        columnKey,
        newValue,
        existingValue);
  }

  // redeclare to satisfy our test for b/310253115
  @Override

import java.util.SortedSet;
import org.jspecify.annotations.Nullable;

/**
 * A {@code SetMultimap} whose set of values for a given key are kept sorted; that is, they comprise
 * a {@link SortedSet}. It cannot hold duplicate key-value pairs; adding a key-value pair that's
 * already in the multimap has no effect. This interface does not specify the ordering of the

      for (Object o : entries) {
        @SuppressWarnings("unchecked")
        Multiset.Entry<E> entry = (Entry<E>) o;
        checkArgument(
            elements.add(entry.getElement()), "Duplicate keys not allowed in EntrySetGenerator");
        for (int i = 0; i < entry.getCount(); i++) {
          contents.add(entry.getElement());
        }
      }

        return buf.toString().trim();
    }

    /**
     * Replaces duplicate hosts in the URL using the duplicate host helper.
     *
     * @param url the URL to process
     * @return the URL with duplicate hosts replaced
     */
    protected String replaceDuplicateHost(final String url) {
        try {
            // remove duplicate host

            return matcher.group(1);
        }
        return null;
    }

    /**
     * Gets a duplicate URL by adding or removing a trailing slash.
     *
     * @param requestData the request data to create a duplicate for
     * @return the request data with the duplicate URL
     */
    protected RequestData getDuplicateUrl(final RequestData requestData) {
        final String url = requestData.getUrl();

        projects.add(project2);

        assertThrows(
                DuplicateProjectException.class,
                () -> new ProjectSorter(projects).getSortedProjects(),
                "Duplicate projects should fail");
    }

    @Test
    void testMatchingIdsAndDifferentVersions() throws Exception {
        List<MavenProject> projects = new ArrayList<>();

				for idx, column := range columns {
					if field := sch.LookUpField(column); field != nil && field.Readable {
						fields[idx] = field
						if count, ok := matchedFieldCount[column]; ok {
							// handle duplicate fields
							for _, selectField := range sch.Fields {
								if selectField.DBName == column && selectField.Readable {
									if count == 0 {
										matchedFieldCount[column]++
										fields[idx] = selectField

        this.keys = Arrays.copyOf(keys, newCapacity);
        this.values = Arrays.copyOf(values, newCapacity);
      }
    }

    /**
     * Associates {@code key} with {@code value} in the built map. Duplicate keys, according to the
     * comparator (which might be the keys' natural order), are not allowed, and will cause {@link
     * #build} to fail.
     */
    @CanIgnoreReturnValue
    @Override

Then we can declare `tags` as a set of strings:

{* ../../docs_src/body_nested_models/tutorial003_py310.py hl[12] *}

With this, even if you receive a request with duplicate data, it will be converted to a set of unique items.

And whenever you output that data, even if the source had duplicates, it will be output as a set of unique items.

And it will be annotated / documented accordingly too.

      for (Object o : entries) {
        @SuppressWarnings("unchecked")
        Multiset.Entry<E> entry = (Entry<E>) o;
        checkArgument(
            elements.add(entry.getElement()), "Duplicate keys not allowed in EntrySetGenerator");
        for (int i = 0; i < entry.getCount(); i++) {
          contents.add(entry.getElement());
        }
      }