Quote from Amazon S3 FAQ about durability:
Amazon S3 is designed to provide 99.999999999% durability of objects over a given year. This durability level corresponds to an average annual expected loss of 0.000000001% of objects. For example, if you store 10,000 objects with Amazon S3, you can on average expect to incur a loss of a single object once every 10,000,000 years
These numbers mean, first of all, that they are almost unbeatable. In other words, your data is safe in Amazon S3.
Thus, the only reason why you would need to backup your data objects is to prevent their accidental loss (by your own mistake). To solve this problem Amazon S3 enables versioning of S3 objects. Enable this feature on your S3 bucket and you're safe.
ps. Actually, there is one more possible reason - cost optimization. Amazon Glacier is cheaper than S3. I would recommend to use AWS Data Pipeline to move S3 data to Glacier routinely.