It is totally correct word order to also say duolingo sometimes is strange. just like you can say Sometimes duolingo is strange, etc. The adverb does not have to be right next to or before verb it is modifying. Who is doing the English language checks for acceptable word order?
While others up there argued that "Duolingo sometimes is strange", I support your idea. In that sentence that you've proposed, the emphasize is in the "is" and "strange". So this can't really be classified as incorrect. English is a very flexible language, many uncommon grammars have been accepted from time to time.
While we all agree in content that indeed Duolingo sometimes is strange...there is also nothing grammatically or syntactically wrong with this statement...nor is it that awkward. In fact, some literary giants would have phrased it that way for certain emphasis. "Indeed, Duolingo sometimes in strange...very strange."