Experiments experience diminishing returns: the more times you run the same experiment, the less Science you receive from it. This prevents you from breaking the game by, say, strapping 500 Mystery Goo Containers onto a rocket, firing it five feet off the ground, then recovering it.
The reason the game says both Goo Containers will net you 15 Science is because, as Ross Ridge said in the comments, it doesn't know which one will get recovered first (if at all) until you actually recover the craft.
The Science Center building in the KSC keeps track of which experiments you've run in which locations - this is available from the second tab (thanks @Philipp). You can consult this before launching a new mission to make sure you're not repeating any experiments you've already done.
The simplest way to avoid repeating experiments during the same mission is to only take one of that experiment - most are so light that they aren't going to significantly affect your craft's balance. Alternately, you can run each of the experiments in a different biome, which will count as two separate results (provided you haven't run that experiment in that biome before).