The code snippet will check if both email field are matched or not

fluent_forms_confirm_email.php

 add_filter('fluentform_validate_input_item_input_email', function($errorMessage, $field, $formData, $fields, $form) {

    $target_form_id = 8;
    if($form->id != $target_form_id ) {
     return $errorMessage ;
    }
    
    
    if( $formData['email'] !=  $formData['email_1']  ){
        $errorMessage=  ['Error! Email does not match'];
    }
    
    return $errorMessage;
}, 10, 5);