X problem in apt-get upgrade

[Richard Kimber <rkimber@ntlworld.com>]

I run testing, with a few gtk2-related packages from unstable

Today apt-get upgraded X.  It held back x-window-system-core and gave the
following:

Preparing to replace xfree86-common 4.2.1-3 (using
.../xfree86-common_4.2.1-6_all.deb) ...
Unpacking replacement xfree86-common ...
dpkg: error processing
/var/cache/apt/archives/xfree86-common_4.2.1-6_all.deb (--unpack):
 trying to overwrite `/usr/bin/X11', which is also in package realplayer

.... and
E: Sub-process /usr/bin/dpkg returned an error code (1)

I don't understand enough to know what has happened and what I should do
about it.  In the past I've had problems getting X working, so I'm
inclined to panic, but will wait to see if anyone can offer me some advice
first :)

Thanks,
- Richard.
-- 
Richard Kimber
http://www.psr.keele.ac.uk/