They are already doing some deduplication. From https://spideroak.com/whyspideroak :

> Greatly reduce backup & sync time through comprehensive compression and advanced de-duplication (saving you time)

> You are only charged for the compressed de-duplicated data amount (saving you money)

Still it is not clear if they do cross-user deduplication, but I think it is very unlikely because all the content is encrypted with an user-specific key, which I think they don't have access to.