Some favorite playlists are restored as empty from backup file #621

Brief summary

Some favorite category playlists are restored empty from the backup file. While some are restored normally, others are displayed as empty.

It seems that the bug happens when the same manga is saved in more than one playlist at the same time

Steps to reproduce

Create different categories and put any stuff there (try saving to more than one playlist at the same time)
Backup
Restore on another phone

Kotatsu version

6.5.4

Android version

13

Device

No response

Acknowledgements

I have searched the existing issues and this is a new ticket, NOT a duplicate or related to another open issue.
If this is an issue with a parser, I should be opening an issue in the parsers repository.

### Brief summary Some favorite category playlists are restored empty from the backup file. While some are restored normally, others are displayed as empty. It seems that the bug happens when the same manga is saved in more than one playlist at the same time ### Steps to reproduce 1. Create different categories and put any stuff there (try saving to more than one playlist at the same time) 2. Backup 3. Restore on another phone ### Kotatsu version 6.5.4 ### Android version 13 ### Device _No response_ ### Acknowledgements - [X] I have searched the existing issues and this is a new ticket, **NOT** a duplicate or related to another open issue. - [X] If this is an issue with a parser, I should be opening an issue in the [parsers repository](https://github.com/KotatsuApp/kotatsu-parsers/issues/new/choose).

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