Fix handling of duplicate dependencies

[valgur]

Duplicates in resolved dependencies are uncommon, but one such case is ffmpeg and libswscale-dev, where both depend on the libswscale-dev apt package. Trying to install them simultaneously fails with

ERROR: the following rosdeps failed to install
  apt: Failed to detect successful installation of [libswscale-dev]
  apt: Failed to detect successful installation of [libswscale-dev]

This appears to be caused by debian._read_apt_cache_showpkg() assuming a roughly one-to-one correspondence between the packages input list and the output lines from apt-cache showpkg. Due to the latter ignoring any duplicates, the resulting list of dependencies to be installed gets corrupted and some of them will fail to be installed.

As a fix, this PR removes any duplicates in the list of resolved dependencies (without affecting the order).