I noticed that the economics proposal was marked as a duplicate of Economics (Update: no longer listed as a possible duplicate), which seems reasonable if you don't check the link, and I almost voted to close as well. However, I checked and noticed that the latter is actually the closed Economics beta site, which does not seem like a valid duplicate.
Duplicate voting is supposed to be to indicate that the community that is being created in Area 51 already has a place on the network, either an older more established proposal or a site. It doesn't make sense to close things as duplicates of closed sites or closed proposals as those aren't places to build/join a community. Furthermore, in both the case of closed betas and proposals closed after 1+ year in either phase, the participants are explicitly encouraged to try again. For other closure reasons, if the old proposal was really not viable or a content violation, then the new one can be closed for the same reason, and if the old one was closed for being poorly defined then there's no reason to close it.
This was already discussed previously when a similar incident occurred related to the closed Big Data site. Admittedly, it's infrequent enough that the moderators and high rep users can handle it, but I can't see any reason for closing as duplicates of a failed site or proposal should ever be allowed, as it can only be used incorrectly or abusively, and the banner attached to the proposal marking it as a possible duplicate may dissuade others from participating even when they would otherwise be interested.
As such, I request that the system not allow such votes in the first place. "Duplicate of proposal" votes should check that the proposed duplicate is an actually an active proposal which doesn't correspond to a closed proposal or a failed beta site.