Ahmad Otak stabbed his ex-girlfriend’s sister, 17-year-old Kimberley Frank, and her friend Samantha Sykes, 18, in March.
Otak, 21, who is from Afghanistan and entered the UK in 2007, had visited Kimberley’s flat in Barden Road, Wakefield, to discuss his stormy relationship with her sister Elisa before the murders took place.
Otak, of Wakefield, was warned by a judge at Leeds Crown Court he now faces a life sentence.
Judge Peter Collier told him: “You have pleaded guilty to two offences of murder.
“As I’m sure you will understand, the sentence for murder is fixed by law and the sentence that will be passed on you in due course is one of life imprisonment.”
Appearing via video-link from Wakefield Prison, Otak pleaded guilty to two counts of murder but denied two further charges relating to the kidnap and false imprisonment of Elisa Frank.
Richard Mansell, prosecuting, said the pleas were acceptable to the Crown and will be left on file.
He said: “Since they represent an aggravating feature of the two murders, the Crown see no reason to try Otak on these counts.”
Otak also pleaded guilty to carrying a blade, namely a kitchen knife, in Wakefield on the day of the murders.
Steven Harratt 57 , of Wakefield, is accused of assisting an offender by giving Otak food, money, clothing and a satellite navigation system after the murders.
He denies the charge and was granted bail until his trial, the date of which is yet to be fixed.
Otak was remanded in custody and will be sentenced on November 9.