Close button shows two icons where one should be

Close button shows two icons where one should be

I am working on a Oracle Apex model window. I notice that the normal close button has en extra icon next to it.
So I changed the " ui-icon-closethick" to "ui-icon-alert". It seems its also selecting
the next icon ( in this case: " ui-icon-info" ). Could someone explain how to solve this?

Button text:
          <button type="button" class="ui-button ui-widget ui-state-default ui-corner-all ui-button-icon-only ui-dialog-titlebar-close" role="button" aria-disabled="false" title="Close">
            <span class="ui-button-icon-primary ui-icon ui-icon-alert"></span>
            <span class="ui-button-text">Close</span>

          </button>

Screenprint of the button: