Literally: to look alike as 2 water drops.
Meaning: to be as alike as two peas in a pod.
Origins: 16th century, but maybe as old as the 15th century. Originally this expression was se ressembler comme deux gouttes de lait (to look alike as 2 drops of milk.) Milk and water being 2 l liquids, the expression evolved as to use the water.
Synonyms: ressembler à s'y méprendre; Se ressembler trait pour trait.
Il ressemble comme deux gouttes d'eau à son père. ― He's the spitting image of his father.