They're not duplicated per se, we just introduced new universal delimiter for backups starting from version 6.0, nothing will change in terms of functionality or performance of the software.
This change allows to migrate local/cloud backups effortlessly without performing any changes to the data itself. Snowball import is also much more streamlined due to these changes.

Thank you

Will the old : versions of files disappear eventually, or will they stay in place? Is there anything we can do to combine the $ folders and : folders into a single folder? I realise that for normal day-to-day use this is all transparent to the user in the client, I'm just thinking about seeding via other computers or via Snow/Fire/Wasabi balls, particularly for hybrid backups (backup to local NAS plus cloud service).

Thanks

The old versions will be eventually purged according to your retention policy. You can also start from scratch if they are no longer needed. Starting from 6.0 the process of seeding is actually much more streamlined due to these changes, especially for Snow/Fire/Wasabi balls.

In general, there shouldn't be any conflicts when moving data containing both delimiters.