I don't think that it is redundant. I can change the implementation in two different ways with each one breaking only one of the two tests.
Furthermore, the names of the tests explicitly state two different parts of the specification of that decorator:

1. The decorated builder is used to create the object.
2. `NSubstituteBuilder` returns the result from the decorated builder.

Using only the test `CreateWithAbstractionRequest_ReturnsResultFromDecoratedBuilder` would make the first statement implicit rather than explicit.