The question which has been answered in some other website should be marked closed in Stack Overflow? It can help to find active questions and reduce time to investigate duplicate questions with other sites. Also, I feel if the owner creates duplicate questions in other sites, then this information could be added in Stack Overflow, so as if it is answered in any other site, could be marked as closed in Stack Overflow.
One such example that I just encountered is Running Keras model for prediction in multiple threads. I agree that giving links is a bad answer, but unless the question is confirmed to be closed, it will not be clear for new users if they should investigate in the question that has been open since 3 years.