I found a posting by @kenkendk which has some background info about why this is done:
Yes, AWS S3 bucket names need to be globally unique. But don’t you have to precreate a bucket in the S3 console? If so you already have chosen a name, and S3 enforces global uniqueness at that point.
Unless Duplicati has the ability to create buckets (not sure), it doesn’t make sense to me for it to do this prompt. If it does have the ability to create buckets, maybe it should first test for its existence. If it exists, use verbatim. If it doesn’t exist, then maybe displaying a prompt makes sense.
@kenkendk any other thoughts?