If you read the help on that error message you'll see it's quoting directly from section 3.3 of the Atom spec:
"Date values SHOULD be as accurate as possible. For example, it would be generally inappropriate for a publishing system to apply the same timestamp to several entries which were published during the course of a single day."
With a date format that is perfectly capable of representing the time with millisecond precision, the chances that two entries in a feed have the exact same time are fairly low, unless you're not producing very accurate times (which certainly seems to be the case with that feed).
While this isn't strictly an error, it is discouraged, which is why you're seeing a warning and not an error.
Julian Reschke wrote:
In <http://feedvalidator.org/check.cgi?url=http%3A%2F%2Fgreenbytes.de%2Ftech%2Fwebdav%2Fwebdav-ietf.atom>, why is the validator complaining about atom:updated elements to be the same...? After all, both entries have different atom:id elements...
