Oh gosh, out of all such Duo errors, this may be the most ridiculous one that I've seen =='
The problem is, that technically "I'd" can be an abbreviation for "I had" as well, like "I'd done it". Obviously informal, but used. And Duolingo algorithm accepts it automatically, because in some contexts it works. But the algorithm doesn't recognize context, doesn't see that it makes absolutely no sense here. But accepting is one thing, I will never understand why it gets suggested sometimes :/ Anyway, there's nothing we can do on our side.
Maybe in British english you could say 'I'd done it", but as a native American I would classify that along with the absolutely unforgivable British expression of 'Have you got?! ;-) I would argue 'technically' that you can not abbreviate it so, but that is more a question for your English language students than for me. ;-) Thanks for the quick response.