After his twin brother dies, Roman (Dylan O’Brien), an irascible young man living in Portland, becomes emotionally unmoored. What is life without his other half? So far not great. The funeral for Rocky is an awkward affair. Mourners line up to offer Roman and his mother (Lauren Graham) condolences only to start crying the moment they come face to face with the living twin. He looks so much like his brother, they say, it’s weird and a little uncanny.

Premiering at Sundance, James Sweeney’s sly dark comedy Twinless might inspire similar sentiments. The film boasts an off-kilter tone — think the absurdity of Problemista meets the wry sensibility of The Feeling That the Time for Doing Something Has Passed — that shocks before it settles. Like Julio Torres and Joanna Arnow, Sweeney walks a tightrope, shifting between dry humor, gutting devastation and emotional poignancy with varying degrees of success. But even when Twinless falters,...