Uzugami - Bunretsu Zoushoku Ningen

Satoru Takeichi, also known as Ichigo, works at a sawmill. While working, he got into a mechanical problem and ended up having the first joint of his right little finger amputated. He and other employees searched for the cut off part, but couldn't find it. Ichigo goes straight to the hospital and receives treatment. However, when he returned to his apartment after being hospitalized for a few days, he found another version of him... From there, a strange shared life with another [Nigo] begins...

(Source: Coamix, translated)