In Latin grammar, a double dative is the combination of a dative of reference with a dative of purpose. A common translation is "As a (dative of purpose) to (dative of reference)."
In an example from Caesar (Gallic War 7.50): suis saluti fuit, 'he was the salvation of his men', the dative of an abstract noun (salus, 'salvation') expresses purpose while the dative of reference expresses the person or thing affected (sui, 'his [men]').
The best known example is "Cui bono?" This phrase, taken from Cicero, is usually rendered in English as something like, "Who benefits?", or more literally 'To whose advantage?' The double dative construction sounds unnatural if translated literally, "to whom to an advantage," and is better rendered as "to whom as an advantage."
See also