I generated a new receiving address on CoinBase so that someone can send me BTC. I immediately searched the address on blockchain.info and was quite surprised to see that it already had many transactions going back several months! What is coinbase doing? Why don't they give me a brand new address? How can they keep track of what account to credit if they give multiple customers the same receiving address?
It's likely that only used the address internally, so it was not assigned to a customer. I would expect that from here on out until forever, whenever coins are sent to this address, it will credit your account.