Sometimes the presence of answers on a duplicate question can be distracting. Instead of going to the original, which is likely to have better answers, people might linger on the duplicate.
In accordance with this idea, moderators will sometimes delete answers on questions that are obvious duplicates:
When a dupe is pretty obvious, but not exact enough to merge, sometimes we delete answers so that people are more likely to go straight to the parent of the dupe.
There doesn't appear to be a network-wide policy or site consensus on this practice, though.
Should distracting answers to obvious duplicates be deleted?