This happened when I was in High School. I was 17 and was dating a girl, Kristi who was also 17. Kristi had a twin sister, Kerri. They would often dress the same, have the same hairdos and tried very hard to look the same. People really couldn't tell them apart, but I always could instantly. They would always try to trick me, but I would tell them that I always knew. One time after school we were in Kristi's room and were getting hot n heavy, she was giving me head when we heard a noise in the house. So, I pulled my pants up and she went to see who it was. A few minutes later her sister came in the room locking the door and said not to worry as it was only her sister. I was about to call her out and say "oh, it was you Kerri". But she quickly came up to me, placed her hand on my cock and said "now where were we" as she slid down to her knees and started to pull my cock out. I pulled back a little worried that Kristi would walk in, Kerri pulls me back saying that she wants to finish what she started. So, I think about it and realized that how could Kerri know what we were doing, unless Kristi told her. So, I said "where is Kerri"? "Oh, she is downstairs watching tv". At that point I decided to play along and have some fun with it. I start teasing her while she sucks, telling her how much better she is today, more than usual. Telling her that I want to do our favorite thing today, but not telling her what it was, eventually telling her to get on the bed in my favorite position for me. First she leaned on the bed, a position we did regularly, I said no, my favorite position, she layed on her back, I start pretending to get annoyed with her, and grabbed her and flipped her over smacking her ass telling her to get on all fours. I then come up behind her and rub my cock against her pussy then told her to hand me the lotion on her nightstand, she does it asking me why. So, I said that I told her at school that I wanted to do our new favorite thing, and again she looks at me confused until I lubed my cock and pressed it against her asshole, she jumped a little bit, but I grabbed her hips and said "come on sweetie, you always say it only hurts for a minute" and started pressing it in. I gotta hand it to her, she bit her lips and took it. I could tell it hurt, but eventually she started bucking her hips back matching my strokes and even started telling me to fuck her ass harder. I came in her fairly quickly, and we layed next to each other spooning. I then whispered in her ear that I loved fucking her ass and that Kristi wouldn't let me. She sat up and said that I was an asshole and smacked me. I laughed and said that I had told both of them I could always tell them apart. I then suggested that I clean up and would then fuck her pussy, she hesitated for a moment and then agreed. So I fucked her a second time and I have to admit, she was an incredible fuck. I never said anything to Kristi that I knew all along and neither did Kerri. We ended up being fwb for a couple of years even after Kristi and I split up. I love twins.