* 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

        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

	errReplicationNoRule                = Errorf("Replication configuration should have at least one rule")
	errReplicationUniquePriority        = Errorf("Replication configuration has duplicate priority")
	errRoleArnMissingLegacy             = Errorf("Missing required parameter `Role` in ReplicationConfiguration")
	errDestinationArnMissing            = Errorf("Missing required parameter `Destination` in Replication rule")

		// show warning message the callback name already exists
		if idx := getRIndex(names, c.name); idx > -1 && !c.replace && !c.remove && !cs[idx].remove {
			c.processor.db.Logger.Warn(context.Background(), "duplicated callback `%s` from %s\n", c.name, utils.FileWithLineNum())
		}
		names = append(names, c.name)
	}

	sortCallback = func(c *callback) error {
		if c.before != "" { // if defined before callback

 * keys according to the above key ordering, and for each key they iterate across the values
 * according to the value ordering.
 *
 * <p>The multimap does not store duplicate key-value pairs. Adding a new key-value pair equal to an
 * existing key-value pair has no effect.
 *
 * <p>Null keys and values are permitted (provided, of course, that the respective comparators

        // Test unregistration
        assertTrue(mockService.unregisterWitness(regId));
        assertEquals(0, mockService.getRegistrationCount());
        assertFalse(mockService.hasRegistration(regId));

        // Test duplicate unregistration
        assertFalse(mockService.unregisterWitness(regId));
    }

    @Test
    void testMockServiceNotifications() {
        // Test notification sending (just logs for mock)

"^file:.*"         → fileSystemClient
"^ftp://.*"        → ftpClient
"^smb://.*"        → smbClient
"^storage://.*"    → storageClient
```

### Services

**UrlQueueService**: URL queue management (FIFO), duplicate detection via `visited()`
**DataService**: Access result persistence, iteration

**Implementations**:
- `UrlQueueServiceImpl`, `DataServiceImpl`: In-memory (default)