A 29-year-old woman presents at 7 weeks of pregnancy with increasing right lower abdominal pain and light vaginal bleeding. She conceived after in-vitro fertilisation and has no haemodynamic disturbance. Transvaginal ultrasonography shows a 7-week gestational sac containing a yolk sac within the uterus, a separate right adnexal gestational sac containing a yolk sac, and moderate free fluid in the pelvis. Which one of the following is the most likely diagnosis?