@MartinSleziak Thanks for noticing. Do you think the migrated question should be closed as a duplicate of Find all polynomials $P$ such that $P(x^2+1)=P(x)^2+1$?

@YuiToCheng That seems reasonable to me.

If there is a worry that the old question might be closed for lack of context, one could consider to closing in the other direction or as a duplicate of some of the linked questions - if they are better as far as context go. (However, now it's not possible to close as a duplicate of the new question, since it has currently no answers.)

Or simply asking the moderators for merging the older post into some of the newer ones.

Anyway, I'd guess in CRUDE (or in Mods' Office) you could get some reasonable advice on this.)

But personally, I would not worry too much about lack of context for the old post. (At least in this case.)

Moreover, if a post is a duplicate target, the prevents it from being deleted. (So even if it's closed, the answers still remain on the site.)