I may have found a solution:

dpkg -i --force-overwrite /var/cache/apt/archives/libxmuu1_4.3.0.dfsg.1-4_i386.deb

and so on for all of:

/var/cache/apt/archives/libsm6_4.3.0.dfsg.1-4_i386.deb
/var/cache/apt/archives/libx11-6_4.3.0.dfsg.1-4_i386.deb
/var/cache/apt/archives/libxext6_4.3.0.dfsg.1-4_i386.deb
/var/cache/apt/archives/libxi6_4.3.0.dfsg.1-4_i386.deb
/var/cache/apt/archives/libxv1_4.3.0.dfsg.1-4_i386.deb
/var/cache/apt/archives/libxt6_4.3.0.dfsg.1-4_i386.deb
/var/cache/apt/archives/libxmu6_4.3.0.dfsg.1-4_i386.deb
/var/cache/apt/archives/libxmuu1_4.3.0.dfsg.1-4_i386.deb
/var/cache/apt/archives/libxp6_4.3.0.dfsg.1-4_i386.deb
/var/cache/apt/archives/libxpm4_4.3.0.dfsg.1-4_i386.deb
/var/cache/apt/archives/libxrandr2_4.3.0.dfsg.1-4_i386.deb
/var/cache/apt/archives/libxtrap6_4.3.0.dfsg.1-4_i386.deb
/var/cache/apt/archives/libxtst6_4.3.0.dfsg.1-4_i386.deb

If that doesn't work I'll try Stephen's solution.