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If you look back to the very beginning of the republic, the same surnames pop up again and again in the lists of Congressmen, top Executive officers, Federal judges, etc.
People can have the same surnames without being related, certainly without being closely related. The smaller and more homogenous a society is (18th Century America as compared to now), the more likely that is to be true.
I'm not aware of any comprehensive studies making this comparison, but I provided what I think is some pretty compelling evidence that it is far more common now than it was then (though, as I made clear, it was never completely unheard of, and the Time article mentions the Adams and the Harrisons as examples of that, while making the point that it is the exception, not a common feature of our political culture).
If you're aware of evidence that suggests otherwise (and what you said doesn't qualify), I'd be interested in seeing it.