अकृच्छ्रे प्रियसुखयोरन्यतरस्याम्

Operational rule vidhi

The words प्रिय and सुख are repeated optionally when they mean 'easily, without any difficulty'.

Word by word (padaccheda)

Right context अकृच्छ्रे saptamī · ekavacanaTarget प्रियसुखयोः ṣaṣṭhī · dvivacanaRight context अन्यतरस्याम् saptamī · ekavacana

Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").
Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).

Full reading

अकृच्छ्रे प्रियसुखयोः अन्यतरस्याम्

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

Commentary

The words प्रिय and सुख are repeated optionally when they mean 'easily, without any difficulty'.
The word कृच्छ्र means 'difficulty and sorrow', अकृच्छ्र means 'without difficulty or sorrow, i.e. pleasantly'. Thus प्रियप्रियेण ददाति, सुखसुखेन ददाति, or प्रियेण ददाति, सुखेन ददाति = अतिप्रियमपि वस्त्व नायासेन ददाति ॥ That is, he gives with pleasure, without feeling it as a trouble. Why do we say "when meaning easily"? Observe प्रियः पुत्र: "beloved son". सुखी रथः ॥

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.