How do I unconfuse dpkg?

[Paul Scott <waterhorse@ultrasw.com>]

How do I clean this up:?

joy:/home/paul# apt-get -f install
Reading Package Lists... Done
Building Dependency Tree... Done
Correcting dependencies... Done
The following extra packages will be installed:
  xlibmesa3-glu
The following NEW packages will be installed:
  xlibmesa3-glu
0 packages upgraded, 1 newly installed, 0 to remove and 1  not upgraded.
2 packages not fully installed or removed.
Need to get 0B/259kB of archives. After unpacking 614kB will be used.
Do you want to continue? [Y/n]
(Reading database ... 114776 files and directories currently installed.)
Unpacking xlibmesa3-glu (from .../xlibmesa3-glu_4.2.1-6_i386.deb) ...
dpkg: error processing 
/var/cache/apt/archives/xlibmesa3-glu_4.2.1-6_i386.deb (--unpack):
 trying to overwrite `/usr/lib/libGLU.so.1', which is also in package 
mesag3-glide2
Errors were encountered while processing:
 /var/cache/apt/archives/xlibmesa3-glu_4.2.1-6_i386.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)


TIA,

Paul Scott