My Husband's Wife by Alice Feeney (and cookie exchange)

Goodreads Summary of My Husband's Wife:

Eden Fox, an artist on the brink of her big break, sets off for a run before her first exhibition. When she returns to the home she recently moved into, Spyglass, an enchanting old house in Hope Falls, nothing is as it should be. Her key doesn’t fit. A woman, eerily similar to her, answers the door. And her husband insists that the stranger is his wife.

One house. One husband. Two women. Someone is lying.

Six months earlier, a reclusive Londoner called Birdy, reeling from a life-changing diagnosis, inherits Spyglass. This unexpected gift from a long-lost grandmother brings her to the pretty seaside village of Hope Falls. But then Birdy stumbles upon a shadowy London clinic that claims to be able to predict a person's date of death, including her own. Secrets start to unravel, and as the line between truth and lies blurs, Birdy feels compelled to right some old wrongs.