Second call to Mapper.CreaateMap redefiens the map from OrderDtoString to Order, but the actual property mappings stay untouched. The order is created with status "InProgress", but the old mapping which returns Status.Complete is still in place, thus the line dto.Status.ShouldEqual(Status.Complete) is true.

ConfigOverride

[Test]
public void ShouldNotAllowToOverrideConfig()
{
    Mapper.CreateMap<OrderDtoString, Order>()
        .ForMember(y => y.Status, x => x.MapFrom(t => Status.Complete));
        
    Mapper.CreateMap<OrderDtoString, Order>();

    var order = new OrderDtoString
    {
        Status = "InProgress",
    };

    var dto = Mapper.Map<OrderDtoString, Order>(order);

    dto.Status.ShouldEqual(Status.Complete);
}