When iterating on a branch involving binary files, it was possible to

hit a case where we would try to upload the source file associated with

a parent diff and hit a conflict. The API returns a DUPLICATE_ITEM error

because we already have a file with that revision, and RBTools would

then crash.
Because of the way file attachment comments work on diffs, it's fine for

us to share the source file attachment for the parent diff across

multiple different diff revisions. We can therefore ignore the duplicate

error.

Followed the reproduction steps for the bug and saw that before this

change rbt post would crash, and after everything worked fine.

Verified that the posted diffs worked as expected.