Google's existing policy has required each sitelink in a campaign to link to a different landing page. Google has this policy because they want each "user to have a meaningfully different experience on the landing page from each sitelink." Google said they "recently noticed an increase in the number of sitelinks created with the same landing page URLs or identical content."
Thus, Google said that they are "beginning more proactive enforcement of the policy." For new sitelinks, Google will immediately enforce the rule but for existing ones, Google will give advertisers a few months to adjust those links, because they know it can take time.
For more details on how to make these adjustments see the AdWords blog.
Forum discussion at WebmasterWorld.