Dropbox increase her size twice - and i think backup to folder syncing with dropbox. Can there be any problems due to this?
Why not backup directly to dropbox destination?
That said, generally syncing multi-file state its not a good idea because between sync start and end the destination storage is not in the consistent state – the sync is not atomic on a directory level.
However in case of duplicacy this will work due to implementation detail: each snapshot is a separate file and each chunk is unique and immutable.
That said - still, consider using dropbox interface in Duplicacy itself.
No dropbox as backup destination in web version. 
Indeed, this seems to be a deliberate decision (which I wholeheartedly agree with; If it was up to me I would have removed Google Drive and OneDrive as well)
Those file sync services behave poorly not only with duplicacy but with other backup tools as well (for example Synology’s HyperBackup simply times out enumerating versions in a backup residing on Google Drive). These services are not designed for bulk storage – they are optimized for simple document sync tasks and there is throttling to prevent abuse.
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Ok. If I understand correctly, it is better not to use Dropbox.
Have 1.7TB of Duplicacy backups alone residing on Google Drive (G Suite Business), working perfectly fine. Also working just fine with Rclone for large media files and I know quite a lot of people use it for “bulk storage” because it works so well.
I’m wondering if this has anything to do with the g-suite versus the google drive for the masses?
Very possible! I gather the exact same API is used but there may be better transaction limits on G Suite.
A lot of people use it in data hoarding circles and usually the only complaint is not being able to upload more than 750GB a day. 
Which, quite frankly, is plenty if you ask me.