problem with vignettes when S4 classes in packages overlap

( A similar problem is also reported by Sebastian P. Luque with
   library(maptools)
   library(trip)
in the vignette as below ).

I am writing a vignette which loads RMySQL and RPostgreSQL. This 
produces the warning:

Loading required package: DBI
Warning in .simpleDuplicateClass(def, prev) :
   A specification for class ?dbObjectId? in package ?RPostgreSQL? seems 
equivalent to one from package ?RMySQL? and is not turning on duplicate 
class definitions for this class

This can be reproduced by running
   R CMD Sweave --pdf Atest.Stex

where the file Atest.Stex has the lines

\documentclass{article}
\usepackage{Sweave}
\begin{document}
\begin{Scode}
library("RMySQL")
library("RPostgreSQL")
\end{Scode}
\end{document}

These warnings only happen in a vignette. They are not produced if the 
lines are entered in an R session.

(Using R version 2.15.1 (2012-06-22) -- "Roasted Marshmallows" on Ubuntu)

Paul