For the vowel of the root dambh there is substituted i as well as ī before san of the Desiderative when it begins with s,
As धीप्सति or धिप्सति ॥ But दिदम्भिषति before the सेट्सन् ॥ The reduplication falls off by 7.4.58.,
दम्भ इच्-च
Adhyāya 7 · Pāda 4 · Rule 56