Additive. Both RDF(S) and OWL semantics are so-called monotonic: new statements can only yield new inferences but never invalidate inferences that were true in the previous state of the data.
If you have a statement that the domain of :p is :A, then every object with an outgoing :p edge is an instance of :A. If you then add that the domain of :p is :B, then every object with an outgoing :p edge is an instance of both :A and :B.
so:domainIncludes works differently from rdfs:domain. Its semantics is disjunctive, i.e. the domain is A or B, not conjunctive. There're discussions on the Web on the differences between so:domainIncludes and rdfs:domain, but as far as I can tell this isn't the main question here.
Best,
Pavel