Is there any way to disable multiple sessions on Facebook? My girlfriend has my Facebook password and I’m afraid that If I'm chatting while she is logged in, she becomes aware of what I'm chatting. Is there any way to force the Facebook prevent someone else from being logged in where an existing session is already active?
This is not an issue with multiple sessions. Your girlfriend doesn't have to be logged in at the same time as you to see your chats.
Chats are now the same as messages in Facebook so anyone logged in to your account (or the account of the person you're talking to) can click on Messages and see your entire chat history.
You'll have to change your password if you don't want her seeing your activities.
I can not find any way to prevent two Login at a same time but there is a solution for your problem by enabling notification on Login to your account.
read this blog: How Can I Know If Someone Else Logs Into My Facebook account ?