If ever there was evidence that prescriptivist maxims are unnatural and ultimately subservient to psycholinguistic priming, this sentence is it. It comes from an email sent in to Andrew Sullivan which he posted online here:
The people on whose doors I knocked on universally described the candidate as thrilling...
The linguistically delicious part is the unnecessary repetition of on. The author appears to be trying to form an NP with a relative clause that would perhaps be better rendered as “The people whose doors I knocked on”. However, still suffering from post grade school linguistics traumatic stress syndrome (PGSLTS), the author is consciously trying to avoid ending sentences with prepositions (but seems to re-analyze the rule to apply to phrases as well); desperate for grammaticality, the author at first valiantly tries to avoid ending the phrase with on by dislocating it to the front of the RC, but then arrives at the verb “knocked” which is probably naturally primed to be followed by a preposition (at least in this context, perhaps treating the verb as a particle-verb, knocked-on), and so tacks on another on, ya know, just to be sure.
Alas! The power of priming wins the day.