Definition of duality:
It is not about socionics duality. You can see this in other typology systems as well. You can see this in non typological systems. Duality always requires opposite partners.
Saying that, people generally relate to people who are similar to themselves. That's why if someone is very similar, same, it is considered as good compability in dating platforms. However, that is not duality.
You can claim that is the best partner for sx/sp. However, you cannot claim that is duality because it is not according to the definition of duality as it is explained above.