"We have made cakes."
Translation:Nous avons fait des gâteaux.
This sentence should always be translated by the passe compose in my opinion, not the imperfect.
Duolingo says I missed a second correct to translation in the multiple choice "Nous faisions des gateaux" But that means something different ie "We were making cakes"
you are referring to an ambiguity that you can find several times in different lessons: Duolingo considers that the simple preterit is a valid translation for the French passé composé as well as for the imparfait, which is rarely true.
I was confused over this, but i found this explanation extremely helpful: http://french.about.com/od/grammar/a/pasttenses_2.htm