If you cannot merge songs into a single album, first check that their album name, artist name, and other properties match.

If these are already the same, note that sometimes “foreign symbols” may cause issues. For example, try renaming album “Mačak u vreći” to “Macak u vreci”. If this successfully combines the songs, you can try renaming the album back into its original name.

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via blog.vucica.net