copy files from one s3 bucket in one account to another - only copy files that the destination bucket does not have

I have 2 s3 buckets, one in one AWS account and one in another account. I only want to copy files from the source that do not have the same name as any file in the destination - is there a particularly effective way to do this? I know with the aws s3 sync command you can specify regex to control which files are copied, but I don't think that would help in my case. Also s3 batch operations don't seem to specify which files you can/cannot copy (at least not in an efficient way)

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