Duolingo when i forget a fada,i have completely changed the meaning of the word, it would requite most people to take a second thought to realise: very good!!!
Duolingo when i write fucking ag an mbean agus an gcailín instead of ag an mbean agus ag an gcailín, i have saved space and said a more speakable sentance: WRONG try again loser
Interesting. I've never used Duolingo myself, although I keep thinking I ought to try Swedish or Catalan just so I can have a better idea of what folk using it for Irish are going through, but I've heard of issues like this before and it strikes me as a serious flaw.
Aidhe, "ag an mbean agus ag an gcailín" is preferable in formal writing, but in speech the single preposition is as common or more so. And the demands of academic prose really aren't something anybody using Duolingo should be too very caught up in. If anything, concern for strict formal grammar at beginner-intermediate stages is the enemy of developing fluency, IMHO.
For that matter, córas an tséimhithe is equally correct, even according to the Caighdeán Oifigiúil. I would have written "ag an bhean agus ag an chailín," as would the native speakers from whom I have my Irish. Would Duolingo have accepted that? I confess I have my doubts.
I admit I've no notion of how to fix this. I don't know if the software can be made flexible enough--in the way a competent human teacher would be--to distinguish between multiple correct possibilities and the purely ungrammatical.
But it's something that stands out to me. I've seen learners developing greater fluency while using Duolingo, so I can't say I believe it's necessarily harmful, but I worry sometimes that it may be giving other folk a bad time and perhaps even discouraging them from keeping on.