However, what seems undersirable for me is to have the answers split (and typicaly people won't crosspost the answers). And, this seems for various reasons counterproductive to me.
I would not like to particularly encourage the following type of behavior, but to get my point accross. If somebody just reposted the question, with an addon this is just for information, if you answer please answer on the other site, I would prefer this over a situation where there is just a double post. [And would have less problem with this.]
So, I see little reason to have the same question open in parallel on both sites.
To put it differently, if I advocate closing one half of a double post, it is not to 'punish' the poster, but simply to organize things; there is also hardly anything lost for the OP. If somebody sees the question and has something interesting to add, I assume under most circumstances the person would make the way to the other site.
To say it still differently: one can also say the mention of the question on the other site is a service to the readers that might otherwise miss out some answers.
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