On general strengthening, or on learning/repeating one particular unit?
From other courses, I've heard reports of "zombie" sentences that were supposedly deleted show up occasionally under circumstances that are not clear to me, but "general strengthening" often seems to be the "culprit".
I'm afraid there's nothing I can do about that -- I'd say it's a bug in Duolingo.
You could try logging out of Duolingo and closing your browser, in case it's got something to do with caching, but I'm not sure whether that would help. (Or if you are using an app, waiting for the next version of the app to be released.)