Rashi on Genesis 4:10/רש"י על בראשית ד׳:י׳

1דמי אחיך THY BROTHERS BLOOD — דמי is plural — bloods” — his blood and the blood of his possible descendants (Genesis Rabbah 22:9). Another explanation of why the plural is used: he inflicted upon him many wounds, because he knew not whence his soul would depart (i. e. which blow would prove fatal) (Sanhedrin 37b).