NCC documents with multiple references to the same SMIL resource fails

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Books that has multiple references to the same SMIL resource in the NCC does play, but navigation using the TOC is broken.

This is caused because the section creation code assumes that the sections have unique urls (not including the anchor reference).

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Steps to reproduce:

• On m.e17.dk (not test.m.e17.dk), log in as 66842, password Nota3913.
• Open book BTKL20130032.
• Open the book index.
• Click Udland:.

Expected behavior: the player should skip to the section Udland. Actual behavior: the player skips to the beginning of the book.

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This is a duplicate of issue #579.