I feel like it's a result of bad UX design if users ask same things over and over again. StackExchange's engineers were quite ingenious by automating answer suggestion and finding possible duplicates even before someone gets to the point of clicking the Post button.
I guess it fell into the StackExchange issue, where beginners or newcomers were not using the search and just asking the same question over and over.
Mods got tired of closing threads, so the users basically self police by marking dupe or telling users to use search.