SurrogateMultiCritLearners should assert during initialization that the list of learners holds unique (w.r.t to the objects) learners

[sumny]

e.g. currently it is possible to do:

learner = lrn("regr.ranger")
surrogate = SurrogateMultiCritLearners$new(learner = list(learner, learner))

During calling $update() both updates will be called but the first learner$model object now "points" to the second trained model because both learners are virtually the same object and not deep clones.