Only create the first instance of a file shared between packages

Only create the first instance of a file shared between packages

- When packages share an identical file, we only need to create the
  first one we encounter and skip it for the rest (reverse already
  happens, and must happen, on erase). This has the benefit of
  getting the timestamp to match the first file, which often is
  also the last one to get removed. Also when lots of shared files
  are involved (such as in multilib installations), this can give
  several percents worth of speed gain as we dont unnecessarily rewrite
  the same contents over and over.