ru is substituted for the n of kān when it is followed by another kān which is an amreḍita (repeated).,
The sutra might have been कान् कानि; but the use of the longer form आम्रेडिते shows, that where there is 'doubling', and the word gets the designation of amredita, then the rule applies. Thus काँस्कान् आमन्त्रयते, काँस्कान् भोजयति; or कांस्कान् &c. When the second कान् is not an amredita, we have कान् कान् पश्यति? Here one is किं asking question, and the other is used in the sense of contempt. This word is read in the list of कस्कादि 8.3.48, and hence sutra 8.3.37, does not apply. Or the स् of समः स्सुटि 8.3.5, is understood here, and that स् is enjoined here and not रु ॥ It should not be objected that in the preceding sutras also स् should be enjoined and not रु; because in those sutras रु is appropriate but not so here.
Why do we say 'when an amredita follows'? Observe कान् कान् पश्यति where one is interrogatory and the other denotes contempt 2.1.64.,