Two Commodities
Two Commodities refers to the idea that in the Dupliverse, there are only two items for which any reasonable sort of scarcity exists, and which therefore can be considered commodities. With the advent of duplicators, any desired object can be produced in any desired quantity, rendering any concept of scarcity meaningless. If any object or substance can be created with equal facility to any other material, then no object or substance is any more valuable than any other. Gold and rubies are as readily available as dirt, and so just as expensive.
Though no material object inherently has significant value in the Dupliverse, however, there are a few items that the duplication itself relies on, and that do have some worth that the duplicated objects do not. For one, duplication requires energy, and the supply of energy is limited. Energy is abundant enough to more or less supply the needs of the Dupliverse's current inhabitants, but it is not unbounded; there are many sources of energy, from nuclear to solar, but they are not inexhaustible nor infinite, and new sources of energy still do have value.
Secondly, the fact that duplicators provide in principle the ability to create almost anything doesn't necessarily mean that just anyone is necessarily going to be able to do it. The duplicators have to be correctly directed and programmed... and, if the intent isn't to make a simple copy of an existing object (and sometimes even if it is), that requires a detailed knowledge of the makeup and features of the object to be made. Certainly creating anything brand new necessitates a good understanding of what it is one is creating.
These, then, are the two commodities of the Dupliverse. Precious stones and metals may have lost their preciousness, unique works of art may be easily made plentiful, life itself may be manufacturable and replaceable, but the commodities of energy and knowledge remain.
History
Although similar ideas had made the economic rounds for many years, the idea of the Two Commodities was first explicitly formulated in 2420 by Althun Doole, in his book The Scarcity of Scarcity. The phrase had its coming out in the universal lexicon four years later when it played a prominent part in a popular novel by Iko De. Now, the idea of the Two Commodities is so ingrained in the culture of most planets of the Dupliverse that the people accept it as an ordinary part of life, and don't particularly question it.
Energy
The pervasive duplication in the Dupliverse requires an enormous amount of energy, which its inhabitants obtain from a number of sources. The largest single source of energy is stellar—the Dupliverse has built huge orbiting solar cells near a number of stars, and even enclosed some stars with no inhabited satellites entirely with photovoltaic Dyson spheres. In other systems, schemes have been implemented to drain rotational energy from uninhabited planets, stars, black holes, and other celestial bodies.
In addition to generating energy, transporting the energy is also an issue. Energy may be produced a long distance from where it is actually needed. For short-range transportation of energy, the transportation method of choice is as simple as an intense, focused electromagnetic beam. For longer distances, so far no better way has been found than to store the energy in some material form (using it to make antimatter is one method) and then physically move the object in question through foldspace, converting it back to energy at the other end.
Knowledge
Duplicators do little good with nothing to duplicate, and knowledgeable operators are needed to program them and to determine the parameters for successful duplication. This is especially true when new or altered objects are created, rather than the exact replication of objects whose characteristics are already stored. In order for such an object to function as intended, the person performing the duplication must be familiar with its workings and understand what effects a given change is likely to have.
Criticism
For all its fame, the two commodities concept has not escaped criticism in the scholarly world. The most frequent criticism is simply that it is trivial. Doole may have coined a phrase that makes a good sound bite, his detractors say, but it contributes nothing significant to economic study. The inclusion of "knowledge" attracts particular scorn, with many economists reluctant to consider such a broad abstraction a commodity on par with energy and other things more easily quantifiable.
Other Commodities
Other critics question not only the utility of the Two Commodities thesis, but its very validity. Even in the Dupliverse, energy and knowledge, they say, are not the only quantities with limited availability. Doole's choice of his two commodities, they charge, was arbitrary; there were several other commodities he could have chosen, including the following.
Matter
Duplication takes not just energy, but also matter—and while energy can be converted into mass, the conversion factor is such (c2) as to make this an impractical method of its generation. Therefore, in a sense matter is also "scarce", in that it is required for the continued operation of the Dupliverse's current civilization, and is available in limited supply. In practice, however, the amount of matter available, in the form of stars and other celestial bodies that do not pertain to inhabited systems, is large enough to make it apparently unlikely that demand for matter will outstrip supply any time soon.
Living Space
Short of multum in parvo warpings of space, the total volume of living space in the universe is large, but finite. That is, even if the universe itself is infinite, the volume that can be colonized within a given time is not. This is especially true when one counts only planets and other habitable bodies, though in that case this commodity overlaps somewhat with matter—after all, planets, too, can in principle be duplicated (even if in practice this is somewhat infeasible with current technologies).
While at the moment there seems to be plenty of room available, if the increase in demand for living space increases at a significantly greater rate—which there is reason to believe it may very well do—then habitable planets may someday be at a premium. Indeed, some economists consider it likely that eventually the greatest scarcity will be in this, and not in energy.
Ideas
Some philosophers have pointed out that not just knowledge is necessary for the creation of new objects, but also ideas as to what to create, and that the two are not synonymous. Knowledge is important, certainly, but so is creativity, and without it simple facts and figures are sterile. Doole's defenders consider this contention hair-splitting, but there are some scholars who see it as a genuinely important distinction.
Nostalgia
Whatever it says about the human psyche, it is undeniable that humans (and most alien races as well) may place value on particular objects that goes beyond their direct utility or the value of their general class. A rock found on a distant planet or a gadget once owned by one's grandfather may be physically identical in every particular to a rock or gadget duplicated from scratch, but for many people the two may not be interchangeable. Thus, in a sense, such original objects have an intangible quality of their own—call it nostalgia—that is in scarce supply. However many copies of things may be duplicated, there will be those who still want the rare originals.