When a vowel does not follow, there is reduplication of yar (all consonants except ha) after a vowel.,
The words अचः and यरः are understood here.
Thus दधि + अत्र = दध्य् + अत्र 6.1.77 = दध्ध्य् + अत्र दद्ध् + अत्र 8.4.53 = दद्ध्यत्र, so also मद्ध्वत्र ॥
Why do we say अचः \after a vowel ?\ Observe स्मितम्, स्मातम् ॥
Vart:- यणो मयो द्वे भवतः ॥ This Vartika may be interpreted in two ways. First taking यणः as ablative and मयः as genitive. \The letters of the pratyahara मय् are reduplicated after यण् letters.\ उल्क्का, वाल्मीकः ॥ Secondly taking यणः as genitive, and मयः as ablative. \The letters यण् are reduplicated after मय् letters\. As दध्ययत्र, मध्व्वत्र ॥
Vart.- शरः खयो द्वे भवतः ॥ This is also similarly explained in two ways. Ist \There is reduplication of खय् letters, after a sibilant for शर् letters\. as स्थ्थाल, स्थ्थात. Secondly. There is reduplication of a sibilant (शर् letters), after खय् letters, ss, वत्स्सरः, अप्स्सराः ॥
Vart:- अवसाने च यरो द्वे भवतः ॥ There is reduplication of यर् when a Pause ensues. As वाक्क्, त्वक्क्, षट्ट्, तत्त् &c.
These reduplications are curiosities, rather than practicalities.,