When creating subscribers through SOAP API, how do ensure that subscribers won't be added as duplicate?
In the Best Practice example code they just ads the subscriber: https://developer.salesforce.com/docs/atlas.en-us.noversion.mc-apis.meta/mc-apis/creating_a_subscriber.htm
The API call will do it automatically for you. If the subscriber key already exists, it will update the same record instead of creating new one
What is a "duplicate" to you? Same unique identifier (SubscriberKey), same email, same cell, similar of the above, similar postal address? How similar? Marketing Cloud matches on SubscriberKey. Anything more sophisticated, do it outside Marketing Cloud.
