[For the record, (I believe) I know the answer, but I saw someone post the question, as stated in the subject, on social media, and I am drawing it to its logical conclusion here.]
Say, for example, that someone makes a pot of Cholent on the stovetop, and, while the Cholent is at a low boil, some powdered milk, of less than 1/60 of the volume of the Cholent, is accidentally spilled into the Cholent and, because of the circumstances, it vanishes from sight almost immediately. A small amount of the Cholent is subsequently eaten, and the Cholent is refrigerated for later consumption.
Let's now say that the chef adds some water to the Cholent, but accidentally adds too much, making it now very watery, and in order to reheat the Cholent, the chef decides it is best to bring it up to a boil and leave it uncovered to allow some of the water to evaporate, thereby thickening it somewhat.
Somehow, another accidental powdered milk spill occurs, this time again with less than 1/60 the volume of the Cholent, but if combined with the first milk spill would now constitute more than 1/60 the volume of the Cholent.
Do we say that it's as if the first milk spill never happened - that first bit of milk ceases to exist in Halachic reality? Or do we say that the first milk spill had no significance to the Cholent at the time of the spill, but with some liquid being boiled off and reducing the overall volume, and some more milk being added to the mixture, do we say that now the first milk and the second milk combine to take up a significant amount of the total volume and make the Cholent not kosher?