UniqueEmail.php

<?php
declare(strict_types = 1);

namespace App\Validator\Constraints;

use Symfony\Component\Validator\Constraint;

/**
 * @Annotation
 * @Target({"CLASS"})
 */
class UniqueEmail extends Constraint
{
    public const IS_UNIQUE_EMAIL_ERROR = 'd487278d-8b13-4da0-b4cc-f862e6e99af6';
    public const MESSAGE = 'This email is already taken.';

    protected static $errorNames = [
        self::IS_UNIQUE_EMAIL_ERROR => 'IS_UNIQUE_EMAIL_ERROR',
    ];
    
    public function getTargets(): string
    {
        return self::CLASS_CONSTRAINT;
    }
}

UniqueEmailValidator.php

<?php
declare(strict_types = 1);

namespace App\Validator\Constraints;

use App\Entity\Interfaces\UserInterface;
use App\Repository\UserRepository;
use Doctrine\ORM\NonUniqueResultException;
use Symfony\Component\Validator\Constraint;
use Symfony\Component\Validator\ConstraintValidator;

class UniqueEmailValidator extends ConstraintValidator
{
    private UserRepository $repository;

    public function __construct(UserRepository $repository)
    {
        $this->repository = $repository;
    }

    public function validate($value, Constraint $constraint): void
    {
        if ($value instanceof UserInterface
            && !$this->repository->isEmailAvailable($value->getEmail(), $value->getId())
        ) {
            $this->context
                ->buildViolation(UniqueEmail::MESSAGE)
                ->setCode(UniqueEmail::IS_UNIQUE_EMAIL_ERROR)
                ->addViolation();
        }
    }
}

UserDto.php

<?php
declare(strict_types = 1);

use App\Validator\Constraints as AppAssert;

/**
 * @AppAssert\UniqueEmail()
 */
class UserDto
{
    ...
}