Updating user's propertySet should not throw exception even if user being updated by two servers in cluster at once

import com.atlassian.confluence.user.UserPreferencesKeys;
import com.opensymphony.module.propertyset.PropertyException;
import com.atlassian.confluence.user.UserAccessor;
import com.atlassian.user.User;
...
/**
     * Updates last login and previous login dates. Contributed by Erkki Aalto and written by Juha Ojaluoma.
     *
     * @author Juha Ojaluoma
     */
    private void updateLastLogin(Principal principal) {

        //Set last login date

        // synchronize on the user name -- it's quite alright to update the property sets of two different users
        // in seperate concurrent transactions, but two concurrent transactions updateing the same user's property
        // set dies.
        //synchronized (userid.intern()) {
        // note: made a few slight changes to code- Gary.
        UserAccessor userAccessor = getUserAccessor();
        User user = (User)principal;
        String userId = user.getName();
        // TODO: Shouldn't synchronize, because that wouldn't help in a Confluence cluster (diff JVMs) for Confluence Enterprise/Confluence Massive. This should be added as a Confluence bug.
        synchronized (userId) {
            try {
                Date previousLoginDate = userAccessor.getPropertySet(user).getDate(UserPreferencesKeys.PROPERTY_USER_LAST_LOGIN_DATE);
                if (previousLoginDate != null) {
                    try {
                        userAccessor.getPropertySet(user).remove(UserPreferencesKeys.PROPERTY_USER_LAST_LOGIN_DATE);
                        userAccessor.getPropertySet(user).setDate(UserPreferencesKeys.PROPERTY_USER_LAST_LOGIN_DATE, new Date());
                        userAccessor.getPropertySet(user).remove(UserPreferencesKeys.PROPERTY_USER_PREVIOUS_LOGIN_DATE);
                        userAccessor.getPropertySet(user).setDate(UserPreferencesKeys.PROPERTY_USER_PREVIOUS_LOGIN_DATE,previousLoginDate);
                    }
                    catch (PropertyException ee) {
                        log.error("Problem updating last login date/previous login date for user '" + userId + "'", ee);
                    }
                } else {
                    try {
                        userAccessor.getPropertySet(user).remove(UserPreferencesKeys.PROPERTY_USER_LAST_LOGIN_DATE);
                        userAccessor.getPropertySet(user).setDate(UserPreferencesKeys.PROPERTY_USER_LAST_LOGIN_DATE, new Date());
                        userAccessor.getPropertySet(user).remove(UserPreferencesKeys.PROPERTY_USER_PREVIOUS_LOGIN_DATE);
                        userAccessor.getPropertySet(user).setDate(UserPreferencesKeys.PROPERTY_USER_PREVIOUS_LOGIN_DATE, new Date());
                    }
                    catch (PropertyException ee) {
                        log.error("There was a problem updating last login date/previous login date for user '" + userId + "'", ee);
                    }
                }
            }
            catch (Exception e) {
                log.error("Can not retrieve the user ('" + userId + "') to set its Last-Login-Date!", e);
            }
            catch (Throwable t) {
                log.error("Error while setting the user ('" + userId + "') Last-Login-Date!", t);
            }
        }
    }