Why Copy Twitter Followers is stupid idea?
First why you need any tools to do so? Yut can do it by yourself on Twitter without risking your Twitter user!
Here are the steps:
1) Sign in on Twitter with your user
2) Open other Twitter user you want to follow his followers (twitter.com/username)
3) Click on Following
4) You will get list of Twitter users this user follows
5) Click on Follow button for every user you would like to follow
If you already follow one of users, it will show it as "Following"
It is simple, fast and safe!
You cannot follow more than 1000 twitter users a day. You also should keep ratio of followers to following. All that means automatic follow is really limited and will not save you too much time.
Another reason why copy followers will soon disappear is a new Twitter API V1.1 limitations. A new limitation will make harder working with big twitter accounts. With all that whatever "copy followers" are offering (validating quality users, active users and users that most likely will follow back) will simply will be impossible. And to deliver such promises requires too much system resources which associated with cost behind.
Twitter automation tools should be beneficial and time savers. Why would you use such tools if Twitter offers you the same in a natural way.
If you need to find all people who not following you amount ones you following, Twitter automation will help and Twitter.com does not offer such alternative. But following others is simple on Twitter.com and there is no need for any other tools.