The Heir and Spare by Jen Geigle Johnson

When Elizabeth Bennet falls for the enigmatic Arthur Darcy, she’s unaware of one small detail—he has an identical twin with a mind (and heart) of his own.

Elizabeth Bennet expected the usual trials of a house party in the English countryside: social niceties, whispered intrigues, and perhaps the occasional dance. What she didn’t expect was to fall for the enigmatic Arthur Darcy—a man whose sharp wit and guarded heart challenge her in ways she never imagined. But there’s a catch: Arthur has a twin.
Fitzwilliam Darcy, Arthur’s charming yet mysterious brother, has a knack for stirring trouble—and Elizabeth’s emotions. As she gets to know them separately, she will soon be faced with them together. Who is truly courting her heart, and who might be hoping for something more of convenience only.

Caught in a web of mistaken identities, simmering tensions, and irresistible attraction, Elizabeth must untangle the truth before her heart chooses the wrong Darcy—or loses them both.