What Duolingo score do you need for the University of Bologna?

Both engineering pages ask for English at CEFR level B2 and list Duolingo (minimum 100) among the certificates that satisfy it. No subscore minimums are given, and the pages do not state a validity period or a sending method for Duolingo results; the certificate is enclosed with the application.

For comparison, the Civil Engineering page sets IELTS at a minimum of 5.5 and PTE Academic at 59, and the Environmental Engineering page also sets IELTS at 5.5.

On both engineering pages, the English requirement is also met by a B2 "idoneità" from the University of Bologna's Linguistic Centre (CLA-UNIBO) or another Italian university, or by a university degree taught fully in English, at least two years (120 ECTS) long, from an EU/EEA country or another listed country. English certificates or medium of instruction letters from universities in countries outside that list are not admissible.

Requirements vary by programme and change between admissions cycles. The score that binds you is the one on your University of Bologna programme page and in your offer letter, so check it there before you book a test.