Best to just judge people on the content of their arguments instead of some concept of identity.
Identity as a basis for trust is over rated.
When people change their identity it is not possible to judge them on the content of their argument as when they changed their identity they also dissociate themselves with their previous arguments.
A way to do it, that I'm sure is going to be deeply unpopular, is to charge a registration fee. Although this would not completely remove sockpuppets it would at lest impose a cost on them. The fees could be used for the good of the community.
Yes, previous arguments are very relevant to future and different arguments. People should fully live under the burden of their history. Nothing should be forgiven or forgotten.
Forgiveness will only lead to a society without justice.