Please describe the problem:
Currently the mass storage dap plugin doesn't handle duplicates, i.e. handling
the addition of a track that is already stored on the dap.

The attached patch adds the following duplicate logic. If the track currently
held on the dap is older than that being added it is removed and the new track
is added. If the added track is not newer then the addition is skipped.

Steps to reproduce:
1. Add a track to a mass storage dap.
2. Add the same track to mass storage dap again.



Actual results:
Banshee crashes.

Expected results:


Does this happen every time?
Yes.

Other information: