$mepr_user = new MeprUser( $user_id );

if ( $mepr_user->is_already_subscribed_to( $product_id ) ) {
    return;
}

// Create the MemberPress transaction
$txn             = new MeprTransaction();
$txn->user_id    = $user_id;
$txn->product_id = $product_id;
$txn->txn_type   = 'subscription_confirmation';
$txn->gateway    = 'manual';
$txn->create_free_transaction( $txn );