2FAuth/app/Rules/CaseInsensitiveEmailExists.php

<?php

namespace App\Rules;

use Closure;
use Illuminate\Contracts\Validation\Rule;
use Illuminate\Contracts\Validation\ValidationRule;
use Illuminate\Support\Facades\DB;

class CaseInsensitiveEmailExists implements ValidationRule
{
    /**
     * Run the validation rule.
     */
    public function validate(string $attribute, mixed $value, Closure $fail) : void
    {
        $user = DB::table('users')
            ->whereRaw('email = ?' . (config('database.default') === 'sqlite' ? ' COLLATE NOCASE' : ''), [strtolower($value)])
            ->first();

        if (! $user) {
            $fail('validation.custom.email.exists')->translate();
        }
    }
}