Great question! When you send a campaign using the Review & Send button instead of the Send Test button, the campaign is moved to the Sent tab. The reason is so that the version of your campaign you sent is preserved exactly as it went out to your audience.
A copy is created when you click on a campaign from the Sent tab so that you don't disrupt the version that was sent. You can work from that copy if you'd like, but it sounds like what your workflow might be is that you're sending a campaign via Review & Send to check out how it looks, and then when you want to keep working on it, you find it on the Sent tab and a copy is created. If that's the case, you'll want to instead use the Send Test tab, which keeps your campaign on the Draft tab without creating copies.
If you're wanting to simply view the campaign you sent -- say, you can't remember the main image you used -- I suggest going to the Response page, clicking on the campaign and then clicking the online version in the lefthand column. You can view the campaign without interfering with what was created on the Campaigns page.
I hope this information helps, but please let me know if I'm not quite hitting upon your process!