Fluent API allows setting the return value twice

[stakx]

This is currently (as of the current tip of master, a964fa7) possible:

public class Foo
{
    public override string ToString() => "original";
}

var mock = new Mock<Foo>();
mock.Setup(m => m.ToString())
    .Callback(() => { })
    .Returns(() => "changed")
    .Callback(() => { })
    .CallBase();

Assert.Equal("original", mock.Object.ToString());

While apparently, the right thing happens, this shouldn't be possible: A method cannot return twice.

[stakx]

This will be tracked in #668 along with some other closely related issues.