Fragmentation with restore multiple channels

Recently I am facing a problem with fragmentation. A supplier is arguing that the way we did a restore using RMAN with multiple channels is causing many fragmentation on the database, and consequently their application on my company is running slowly.

The restore has been done using 6 channels, and I guess that is not true. The database is being used for testing purposes only, and during this phase it's not happening updates.

Anyways, I did some research and I couldnt find a resonable answer. So I am in doubt if restore in multiple channels really causes fragmentation.
Does someone have any experience on that??

Thank you for sharing.
Alex

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