Queuing tracks from a playlist should preserve duplicate entries

[thejoshwolfe]

Steps to reproduce:

1. Create a playlist that contains two entries that are the same file in the library.
2. Queue the playlist.

At this point, you'll see that only one entry was added to the queue instead of two.

It seems like queueSelection() is collapsing the selection into a set of library ids before determining the sort order, which is where the duplicates are being lost.