You need to determine if you are using BES or BIS. BES is the enterprise e-mail handler. This means that you are on a corporate account. If this is the case, contact your IT administrator. If you are an individual and not using a corporate connected BB, then you would be using BIS (Blackberry Internet Service) as your mail handler. You need to goto the following website:
https://bis.na.blackberry.com/html?brand=vzw
This is the specific page for Verizon customers. If yo have set up an account, you just go and cancel the duplicate copy. If you have not set up an account, do so and when you do, make sure that you do not select the "send a copy" option. This is the only way to correct he problem you are having. 73