This is the response to the general question about duplicates of duplicates. (I.e., I am not commenting on the specific questions linked above as an example.)

I can see several situations where I might consider choosing as a duplicate target a question which already is closed as a duplicate:

• One reason is to add a link to question with useful answers. For frequently asked questions, there are usually several copies floating around. If a question is closed with more than one duplicate target, this might help users who stumble upon this question links to several interesting answers. (Admittedly, there is not much difference if the alternative link is only mentioned in comments - which adds the post among linked questions.)
• If I think that Q1 is duplicate of Q2 but I disagree with the earlier closure of Q2 as a duplicate of Q3, then Q2 is the most logical duplicate target for me. (Of course, in such situations I should also try to reopen Q2 if possible.) Here I can include even more specific example. Personally, I would prefer not to close question about $\sum_{k=1}^\infty \frac1{2^k}$ as a duplicate $\sum_{k=1}^\infty x^k$, for the reasons I tried to outline in this answer. However, it seems that many other users have different opinion on this. Still, if there as a new question about this, it might be more useful for the asker if they are directed via duplicate link to the question about the special case $x=1/2$ rather than to the more general question - even in the case that the special case is closed as a duplicate of the general case. (Probably ideal situation is that the asker is given both links. However, if we assume that it is possible that the duplicate from special case to the general case might be cancelled in the future; it is better to have among duplicate targets at least one question which makes sense even after this - hypothetical - change.)

This is the response to the general question about duplicates of duplicates. (I.e., I am not commenting on the specific questions linked above as an example.)

I can see several situations where I might consider choosing as a duplicate target a question which already is closed as a duplicate:

• One reason is to add a link to question with useful answers. For frequently asked questions, there are usually several copies floating around. If a question is closed with more than one duplicate target, this might help users to get links to several interesting answers - more of them than if only one duplicate link is given. (Admittedly, there is not much difference if the alternative link is only mentioned in comments - which adds the post among linked questions.)
• If I think that Q1 is duplicate of Q2 but I disagree with the earlier closure of Q2 as a duplicate of Q3, then Q2 is the most logical duplicate target for me. (Of course, in such situations I should also try to reopen Q2 if possible.) Here I can include even more specific example. Personally, I would prefer not to close question about $\sum_{k=1}^\infty \frac1{2^k}$ as a duplicate $\sum_{k=1}^\infty x^k$, for the reasons I tried to outline in this answer. However, it seems that many other users have different opinion on this. Still, if there as a new question about this, it might be more useful for the asker if they are directed via duplicate link to the question about the special case $x=1/2$ rather than to the more general question - even in the case that the special case is closed as a duplicate of the general case. (Probably ideal situation is that the asker is given both links. However, if we assume that it is possible that the duplicate from special case to the general case might be cancelled in the future; it is better to have among duplicate targets at least one question which makes sense even after this - hypothetical - change.)