If you get a request to accept a person as a friend but yet you already have that person as a friend, Chances are it is a bogus account that a hacker is using. However, some times a person may have more than one account.

If you get a friend request from an existing friend, check with that friend to see if they actually created the account. If they didn't create the second account, informed them that their is a second account of them and they need to inform Facebook and their friends.  

The hacker will usually steal your photos and take note of who your friends are. They will then create another Facebook page with your name on it, will use your photos so it will look like your page and then send friend's request to your friends.

Note: You may have your security settings set so people cannot download your photos. However if the hacker is able to see your photo, they can get your photos even if you have the download feature disabled. There is tools out there that can make copies of anything that you see on the screen. Windows has that feature built in. It is called the Snippet tool. For more information on this tool, click here.