Recently I noticed strange behavior when viewing posts "marked as duplicate" on certain Stack Exchange sites. Here are some examples:
These questions above show up properly with "marked as duplicate" sign if I currently logged in and joined the community at the respective sites. However, if I logged out from the current session or using another browser without logging in, those pages sometimes redirected the link request to their duplicate sources instead of showing requested post (at respective order given above):
Note that not all of duplicates have a similar kind of behavior, as I had tested, multiple duplicate sources doesn't have that (an example from Travel.SE: Is it hard for me getting the tourist/visitor UK or US visa?).
Here's I want to ask then:
Why do some "marked as duplicate" posts redirect to other existing post as their duplicate source instead of requested post itself, especially when not logged in?
Is that kind of behavior intentional, or currently is it a "glitch" (i.e. "bug") when a redirection process to another post is taken instead of directly showing the requested post link like other non-duplicate posts?
NB: At this time I can reproduce on 4 sites (including this MSE). It is possible the same behavior exist on other Stack Exchange sites not covered here.
Update: Stack Overflow also has this behavior:
which redirected to this duplicate source: