Until recently. Now it seems things are different.
Attack of the Clones?
It's undeniable, for example, that there are similarities between the DnB NOR Headquarters by MVRDV (Oslo, Norway, 2003), the Rotterdam Stadskantoor of OMA/Koolhaas (Rotterdam, The Netherlands, 2009) and the XINA Hangzhou Waves by JDS/Julien De Smedt Architects (Hangzhou, China, 2011). Or between the Vibenhus Office Building by BIG (Copenhagen, Denmark, 2006) and the Binhai Mansion by OMA/Koolhaas (Shenzhen, China, 2011). Or between the Vilnius WTC by BIG (Vilnius, Lithuania, 2007) and Future Towers by MVRDV (Pune, India, 2011). The possible similarities are not limited to formal approaches. Further analogies can be found in the designers' methods of representation. In the competition for the Rødovre Skyscraper in Copenhagen in 2008, in addition to the recognisable three dimensional pixels, the winning project by MVRDV and ADEPT used some diagrams with compositional schemes already typical of a presentation by PLOT. Chance would have it that Bjarke Ingels's studio also entered the competition, but was beat out by its own weapons of communication, as it were. So what's happening? Are these courteous, more or less conscious exchanges and tributes among esteemed colleagues? Or are we dealing with poorly veiled copies or clones, perhaps justified by the short time frames imposed by competitions? Perhaps the real question is: does absolute originality, "a pure creative act", actually exist in architecture?
The issue is complex, and it has to do with the transmission of culture, taken as a system of ideas, theories, beliefs and instructions, which also includes architectural language and its approach to space and form. In today's world, ideas, theories, conventions and instructions are transmitted differently than they were in the past. Even architecture, which has always been considered the "longest" of the arts, and afforded a wide berth for the incubation of thoughts and reflections—in Italy, perhaps a bit too wide—has become something nearly usable in real time.
Someone might ask, "What do the different modes or speeds with which contemporary architectural culture is transmitted have to do with the various 'clones' that have emerged around the world?" The resounding answer is, "A lot." And we're not dealing with clones. Culture, even architectural culture, is rarely the direct result of an original quantum leap in thought. Rather, ideas, and human thought in general, arise from the composition / decomposition of pre-existing information that, sooner or later, filters into our collective store of knowledge only to re-emerge later, perhaps in some mutated form.
Someone has tried to trace the similarities between the evolutionary model explaining the transmission of genetic inheritance in living organisms and the way cultural evolution occurs. In the same way that genes are understood to be the basic hereditary unit in all living things, in his 1976 book The Selfish Gene, Richard Dawkins introduced an analogous concept, the "meme", to explain how culture and information propagate. These memes, units of cultural transmission, can utilise us or other media—i.e. all forms of "media storage", both biological and technological—as carriers. It's a fascinating theory that can easily be applied to the field of architecture. 
Memes are ideas or parts of ideas: a language, a cultural custom, a religious belief, an urban legend, a melody, an aesthetic, a theoretical approach, a particular technical—or architectural or formal—solution, etc. They can be transmitted from one mind to another and are associated with each other, acquiring independent living skills and outstanding capabilities of "viral" dissemination and replication. The result, in some cases, is the "repetitive infectious" meme, the so-called "smash hit" or "seasonal fad".
Viva la Evolución! 
The word "meme" is derived from mnimis, Greek for "memory". According to Dawkins's original definition, a meme is "that which is imitated". All information that, when imitated, is subjected to variations and subsequently selected in the evolutionary process "will produce a project" , a step forward. In other words, "a copy with variations and selection". So, compared to the cultural "creationist" model—still widespread in the field, and naturally referred to as "creative"—we found ourselves on the opposite side of the question: more often than we prefer to think, in order to see further one must "stand on the shoulders of giants". Sir Isaac Newton had already noticed this a full two centuries before Darwin. There are "strong" memes, similar to the "dominant characters" in genetics, which possess a marked capacity to spread and replicate (e.g. the classical orders, or the triangular pediment that outlasted the temples themselves, or the latest, ubiquitous tetris voxels), and "weak" memes, with little or no ability to spread: maybe good ideas in themselves, but unsuccessful.
Today, now that ideas and information can travel from one brain to another instantaneously through the Internet—somewhat akin to how viruses can be spread by intercontinental flights—memes have a much greater ability to move around and take root. And so it is with architecture as well. Indeed, it is clear that some architectural solutions work better on a global scale than others, at least in the minds of designers, or contractors, or competition juries, and therefore they "replicate" themselves. Memes that are stronger than the competition, "formal memes" accompanied by equally battle-hardened "theoretical memes", struggle for hegemony until they are superseded by other memes. And so on.
Dissemination, infections, pandemics
So, in defence of the almost embarrassing similarities between some projects such as those mentioned above (but you could find similarities between other projects and other designers, too), it is simplistic to think that we're dealing with a phenomena of trivial emulation or worse, plagiarism. It would be fair to claim, instead, that what we have witnessed in recent years is a process of infectious repetition as seen, for example, in the derezzed/tetris block meme or the "extreme cantilevers" of WoZoCo/MVRDV. The architects had little to do with it. In spite of themselves, they were used as carriers by the dominant architectural memes that have virally crept into the terminals (and minds) of unsuspecting project leaders, with the outcomes that we've seen.
It is obvious that the infection of "architectural memes" occurs prevalently where special conditions exist for the disease to spread. Factors such as educational background, past or present participation in specific working groups or the transfer of cultural assets (partners or employees) between competing studios, when combined with the possibility of sharing information instantly around the globe, further create interesting conditions for "cross-pollination" or "strain mutations" of the hereditary characteristics. In this sense, Rem Koolhaas/OMA/AMO—the studio with the all-time largest number of high-quality spin-offs , actual "memetic laboratories" that constantly produce different theories, ideas and architectural solutions, more or less successful experiments, authoritative strain mutations of the equally authoritative source—is the most imposing incubator of architectural culture in recent decades. A little like what happened nearly a century ago in the studio of Peter Behrens , where "Modern" was essentially born.
In light of these observations, I'll try to formulate a response to the question we posed a moment ago: does absolute originality, the pure act of creation exist in architecture? I would say no. Or maybe it exists, but it's "not pure". We have seen that a form of creative act—certainly not absolute, i.e. coming from nothing—can be found in the action of "imitation with variations and selection" that characterises all evolution. Even more creative, if you will, are leaps forward in the form of a "scale change", or a "paradigm shift". For example, the transfer of concepts from art or music to architecture has always presented a mutually beneficial osmosis (we could talk for hours about the debts owed by much of contemporary architecture to Sol LeWitt or Donald Judd).
Now, if we admit that the discipline of architecture lies within the domain of knowledge that is available and shared, then all theories, or formal solutions that can feasibly be realised in the form of space or a building, represent a more or less. advanced entity in this domain, adapted to the context of the intervention. In other words, we all have references, a background of information that we draw on for our intellectual activities that are their foundation or context.
We always start from the acquired data and move forward (or, in a worst-case scenario, backwards). It is exactly for this reason, as I was saying, that it is wrong think to about a work of architecture as a unicum, conceived in the sublime, self-sufficient cerebral universe of the designer-creator-demiurge.
An experiment was developed to test this argument in the field: which entities in a domain of knowledge, which theories or formal solutions can be classified and correlated with each other? For example, given a sample of internationally recognised architects, relationships can be established between their projects that highlight their evolutionary nature as well as the quotient of continuity or discontinuity compared to their overall output.
The resulting map represents a fragment of a possible "taxonomy in progress" in the language of architecture, in which each "taxon" can be composed of a systematic group of entities that can be traced back to the same theoretical or formal rule (meme?). Obviously, the sample is limited both in terms of time and specimens, and the rules / memes may be extended at will. An analysis intended for print publication of such a complex issue necessarily works within the confines of discretion.
Let's consider it a point of departure. The aim is to set up a sort of annotated knowledge base of projects and architectural artefacts, a wealth of knowledge useful for ordering and understanding their evolutionary lines, their intersections (touch points or crossbreedings) and leaps forward with the addition of new topics and new points of view (milestones), their latent presence or their extinction. Understanding why some of these ideas have been more successful than others. And finally debunk the myth of the lone, supercelestial Creator-of-Beauty, who always knows perfectly well to be in good company. Luca Silenzi (@spacelab_it)
1 The matter is further complicated by the fact that many international studios hire the same talented firms specialised in renderings and pre-visualisation, such as MIR or Luxigon, resulting in uniform graphics and atmospheres among different projects and designers
2 In Italy, the first to deal with architectural memes was Paolo Bettini in his essay Evolution— which was the main inspiration of these discussions—published on the Internet in his must-see lessons
3 Exclamation by Bjarke Ingels in the introduction to his Yes is More, 2009
4 Susan Blackmore, Imitation and the definition of a meme, in Journal of Memetics. Evolutionary Models of Information Transmission, 1998
5 Paul Makovsky, Baby Rems, Metropolis, January 2011
6 Conrad Newel, Work for Rem part II, Notes on becoming a famous architect, 25 March 2011