title: Correct usage of mlr::mergeBenchmarkResults
body: <p>I'm trying to resolve an error I'm facing with <code>mlr::mergeBenchmarkResults</code>, which is:</p>
  
  <pre><code>Error in mergeBenchmarkResults(bmrs = list(bmr, bmr_no_mos)): The following task - learner combination(s) occur either multiple times or are missing: 
  * wnv_no_mos - rpart
  * wnv_no_mos - rf
  * wnv - rf_no_mos
  * wnv - xgb_no_mos
  * wnv - extraTrees_no_mos
  
  Traceback:
  
  1. mergeBenchmarkResults(bmrs = list(bmr, bmr_no_mos))
  2. stopf("The following task - learner combination(s) occur either multiple times or are missing: \n* %s\n", 
   .     msg)
  </code></pre>
  
  <p><strong>In a nutshell, my desire is...</strong></p>
  
  <p>I have two tasks, two learner sets, and two benchmarking objects. I wish to combine the two benchmarking objects using <code>mlr::mergeBenchmarkResults</code>.</p>
  
  <pre><code>tsk = makeClassifTask(data = wnv, target = "y", positive = "Infected")
  lrns = list(makeLearner(id = "rpart", cl = "classif.rpart", predict.type = "prob"),
              makeLearner(id = "rf", cl = "classif.randomForest", predict.type = "prob"))
  bmr = benchmark(learners = lrns, tasks = tsk, resamplings = rdesc, measures = meas, show.info = TRUE)
  
  tsk_no_mos = makeClassifTask(data = wnv_no_mos, target = "y", positive = "Infected")
  lrns_2 = list(makeLearner(id = "rf_no_mos", cl = "classif.randomForest", predict.type = "prob"),
              makeLearner(id = "xgb_no_mos", cl = "classif.xgboost", predict.type = "prob", nthread=25),
              makeLearner(id = "extraTrees_no_mos", cl = "classif.extraTrees", predict.type = "prob", numThreads = 25))
  bmr_no_mos = benchmark(learners = lrns_2, tasks = tsk_no_mos, resamplings = rdesc, measures = meas, show.info = TRUE)
  
  mergeBenchmarkResults(bmrs = list(bmr, bmr_no_mos))
  </code></pre>
  
  <p>What am I doing wrong?</p>