looks like there are two versions of libart available and they contain the same file, so dpkg won't let them be installed at the same time. You can track which package depends on which libart by doing:
apt-cache rdepends libart-2.0-2 to see what depends on it. The dependency list will include all packages that depend on it, not just the ones installed. To see whether a package is installed and what repo it comes from, use:
apt-cache policy package
to search for a package name when you only know part of it:
apt-cache search libart

-John