This has been asked on Meta.SE. See here and here.
Based on those answers: the policy is that it's okay as long as such users don't abuse their multiple accounts. To quote from one of the answers:
If the second account allows you to do something on the site that your
normal account would be prevented from doing, it is abuse. Examples of
this include (...
You can contact the team for merging accounts, see this post for details.
As explained there, visit https://philosophy.stackexchange.com/help/merging-accounts with this account and have the link/user number of the other account ready when doing so (in this case, the account number is 12597321 for the whole network and the user number on Philosophy.SE is ...