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Given a site that publishes unique content online and given that there are lots of site scrapers and persons stealing content and reposting it as their own, inevitably you will have the same content hosted on different sites.
Is there any measure that can be taken to later prove, beyond any doubt, what the original source of the content was?
What you want here is not "non-repudiation". Non-repudiation is about preventing the true author of a document from later on claiming that he was not, in fact, the author, and cannot be held responsible for the document contents.
No, what you seek is quite the contrary: you want to prevent non-authors from claiming that they are the true author. You want to prove that you produced/published the data first. This is called a proof of anteriority. In the non-computer world, this is done with a Soleau envelope (in France). The computer equivalent is Trusted Time-Stamping: an "authority" (called "TSA") applies a digital signature on a structure which contains both the current date (as known by the TSA) and a hash value; assuming that the hash function is cryptographically strong (resistant to second preimages), this signature is a proof that "whatever was hashed" already existed at the date contained in the time stamp. All you have to do is then to time stamp a copy of your content along with an unambiguous designation of yourself (say, your name).
How a time stamp may hold in court, during litigation, is a complex question. It depends on a lot of things. In France again, there is a notion of horodatage qualifié which designates a TSA whose systems apply a lot of security measures which collectively ensure that the time stamp is trustworthy; it is all spelled out in this document (in French). Supposedly, a time stamp from such a TSA would be accepted as proof "by default", the burden of proof being placed squarely on whoever claims that the time stamp is fake. Actual rules are heavy: redundant physical security, regular audits, background checks on all developers and operators... and, last but not least, demonstration that the TSA uses a time source which is accurate to the second, and cannot be easily subverted remotely (e.g. simply using some NTP servers on the Internet is not sufficient, since NTP packets can be spoofed). The "under 1 second accuracy" rule is especially troublesome, because computer internal clocks tend to drift by more than one second per month, and leap seconds cannot be simply ignored, as is customary in computers.
Of course, one would have guessed that France would go for a centralized, bureaucratic set of byzantine rules. In countries with a Common Law tradition (in particular UK, USA, and Commonwealth members), it is expected that whether some time stamps are acceptable as proof "beyond any doubt" or not, will be asserted by judges in the course of the resolution of actual litigation. Which means that question is still, as of now, open.
If you really want, now, to establish your authorship of your works, I suggest that you print them out on actual paper, and use established pre-computer procedures, e.g. registration with a specialized office (of course, there may be fees).
To add to the answer of Tom Leek, the European Union already has a legal framework covering trusted timestamps as part of the definition of a qualified electronic signature in European Directive 1999/93/EC.
Following this regulation, qualified trusted timestamps are also interoperable and have the same legal value in all countries where this notion exists (as of today, 13 Member states have accredited national TSAs to provide such qualified timestamps).
In 2014 a new regulation will establish the legal value and interoperability of qualified electronic timestamps in all Member states.
