Well the host, username and password are not that big deal to be reiinputed.
But since one server might have different session directory to be saved on purpose, it would help a lot to just duplicate the existing one.
I suppose you mean duplicating stored session/site.
In WinSCP 5.2.x: Manage > Clone to New Site followed by Save As.
In WinSCP 5.1.x: Edit followed by Save.
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