"His father had as many pairs of spectacles as mine did."
Translation:Son père avait autant de paires de lunettes que le mien.
The mapping between the various English and French past tenses is not clear cut, and it seems like Duo will accept either in many cases.
This article: https://www.lawlessfrench.com/grammar/passe-compose-vs-imparfait/ lists a number of rules, all of which seem to point to the imparfait for this sentence.
There is also a discussion on the Duo forums here: https://forum.duolingo.com/comment/17567672/Imparfait-vs-Passé-Composé