Why do the participle and verb occur together in ḥāsid idhā ḥasad?

Supplementary Practice 24 — Comparative Rhetoric of Sūrat al-Falaq and Sūrat al-Nās

• Turning both into synonymous prepositions
• Denying that envy may ever do harm
• Limiting envy to feminine sound plurals
• Identifying a disposition and the moment it is expressed