A Food Machine That Dispensed 2 Products for the Price of 1: What Is the Halacha?

Hello, Strictly speaking, you may take the second item without paying for it. Nevertheless, it is good and proper, beyond the letter of the law, to return it to the machine owner. If this happened with a machine located in a Torah-observant place, such as a yeshiva or seminary, there is an obligation to return the item to the machine owner. 

Sources: The basis for the allowance to take the second item is the Gemara in Bava Metzia (21b), which states that it is permitted to take fruit from a tree that is likely to fall, because passersby normally take it, and therefore the owners despair of it initially. Likewise here, it is common for passersby to take the item that came out of the machine by mistake, and therefore the owner despairs of it to begin with. And what was written, that it is good and proper, beyond the letter of the law, to return the extra item to the machine owner, is based on what is explained in Shulchan Aruch, Choshen Mishpat (siman 259, seif 5). In any case, if the machine is located in a Torah-observant area, where the passersby are careful about the prohibition of theft, the machine owner does not despair, and then there is an obligation, according to the strict law, to return the item. This is also written, as stated, in Bichurei Aviv (page 91). 

With blessing, 

Hillel Meirs