2014ish thriller about a London woman in a seemingly stable relationship who goes to meet her boyfriend at airport arrivals one evening but her never shows up. Large part of the novel is her looking for him while a shadowy figure seems to be following her. SPOILERS. Turns out the guy following is his troubled identical twin. Boyfriend shows up and says the brother is dangerous. IIRC he killed his twin’s girlfriend in some kind of violent sex game with drugs while pretending to be him. ENDING The woman is eventually approached by the twin who explains it all and they go to a cottage or somewhere remote. The boyfriend finds out and kills the twin (stabs him I think). The boyfriend then chases the woman in the dark into fields and almost kills her but she kills him with a rock.
Lost Classics of Teen Lit: Name That Book! 