BAD. I am ashamed to talk about it, but if it helps people here stay more secure, it's worth it. So please don't downvote, this could be helpful! I will never do it again.

A few times I really wanted to see someone's Facebook profile, I cloned one of their friend's accounts and sent a friend request. Most of the time, people think their friend has just set up a second account, and don't think twice about adding. I get second friend requests all the time.

First I figured out which of their friends was not close to them, because if they were a close friend then my person would contact them right away and ask why they had a second Facebook account. So it had to be someone they wouldn't be bothered to message.

I looked at their Friends list, which is in chronological order. I didn't want someone from too far back (those tend to be close friends) or too recent, or someone in their country or place of work (likely to be in contact with them). So I picked someone from the middle. (If my person's friend list was hidden, I find names through the likes and comments on their public photos.)

I'd set up a Facebook account under their name, making sure to hide all my contact info etc. Then I'd steal the target's profile and cover photo, and block them. Then I'd send a friend request to my person. As soon as they accepted, I'd see what I wanted to see, maybe save some photos, and then delete the account before anyone got suspicious.

So... Don't accept double requests before checking with that person.

And don't do this, as not only is it immoral but it might be illegal where you live. It is absolutely illegal in most places to impersonate someone online with intent to harm them.