Karwai Tang / WireImage

Lily Allen is lashing out at an unfaithful former partner on her scathing new album , “West End Girl,” out Oct. 24.

Following reports last year that the British singer, 40, had separated from “Stranger Things” star David Harbour, 50, People reported in February that the two had officially called it quits after four years of marriage.

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Now, on “West End Girl,” her first album in more than seven years, Allen chronicles a relationship’s bitter breakdown caused by a partner’s cheating. Across 14 mostly downtempo tracks, Allen weaves together the story of a partner who led a “double life” and violated an “open” marriage code.

On the somber “Sleepwalking,” Allen’s protagonist des

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