diff --git a/R/challengeR.R b/R/challengeR.R
index 7f161d1..53087a2 100644
--- a/R/challengeR.R
+++ b/R/challengeR.R
@@ -1,104 +1,122 @@
 #' Title
 #'
 #' @param object
 #' @param value
 #' @param algorithm
 #' @param case
 #' @param taskName Optional for single-task data set that does not contain a task column.
 #' @param by The name of the column that contains the task identifiers. Required for multi-task data set.
 #' @param annotator
 #' @param smallBetter
 #' @param na.treat
 #' @param check
 #'
 #' @return
 #' @export
 #'
 #' @examples
 as.challenge=function(object,
                       value,
                       algorithm ,
                       case=NULL,
                       taskName=NULL,
                       by=NULL,
                       annotator=NULL,
                       smallBetter=FALSE,
                       na.treat=NULL, # optional
                       check=TRUE) {
 
   object=as.data.frame(object[,c(value, algorithm, case, by, annotator)])
 
   # sanity checks
   if (check) {
 
     if (!is.null(by) && !is.null(taskName)) {
       warning("Argument 'taskName' is ignored for multi-task data set.")
     }
 
     # Add task column for data set without task column by using the specified task name.
     if (is.null(by) && !is.null(taskName)) {
       taskName <- trimws(taskName)
 
       if (taskName == "") {
         stop("Argument 'taskName' is empty.")
       }
 
       object <- cbind(task=taskName, object)
       by = "task"
     }
 
     # Add task column for data set without task column by using a dummy task name.
     if (is.null(by) && is.null(taskName)) {
       object <- cbind(task="dummyTask", object)
       by = "task"
     }
 
     object=splitby(object,by=by)
     object=lapply(object,droplevels)
     for (task in names(object)) {
       # check for missing cases
         missingData=object[[task]] %>%
           expand(!!as.symbol(algorithm),
                  !!as.symbol(case))%>%
           anti_join(object[[task]],
                     by=c( algorithm,case))
         if (nrow(missingData)>0) {
           message("Performance of not all algorithms is observed for all cases in task '",
                   task,
                   "'. Inserted as missings in following cases:")
           print(as.data.frame(missingData))
           object[[task]]=as.data.frame(object[[task]] %>%
                                          complete(!!as.symbol(algorithm),
                                                   !!as.symbol(case)))
         }
       # check duplicate cases
          all1=apply(table(object[[task]][[algorithm]],
                            object[[task]][[case]]),
                      2,
                      function(x) all(x==1))
-          if (!all(all1)) stop ("Case(s) (",
-                                paste(names(which(all1!=1)),
-                                      collapse=", "),
-                                ") appear(s) more than once for the same algorithm in task '",
-                                task, 
-                                "'. Please revise.")
-    #  }
+          if (!all(all1)) {
+            n.duplicated <- sum(all1!=1)
+            
+            if (length(object) == 1 ) { # single task 
+              if (n.duplicated/length(all1) >= 1/5) { # at least a quarter of the cases is duplicated
+                stop ("The following cases appear more than once. Please revise. ",  
+                      "Or are you considering a multi-task challenge and forgot to specify argument 'by'?\n",
+                      "Cases: ",
+                      paste(names(which(all1!=1)), collapse=", ")
+                      )
+              } else {
+                stop ("The following case(s) appear(s) more than once. Please revise.\n",  
+                      "Case(s): ",
+                      paste(names(which(all1!=1)), collapse=", ")
+                      )
+              }
+            } else {
+              stop ("The following case(s) appear(s) more than once for the same algorithm in task '",
+                    task, "'. Please revise.\n",
+                     "Case(s): ",
+                    paste(names(which(all1!=1)), collapse=", ")
+                    )
+
+            }
+          }
 
       if (!is.null(na.treat)) {
         if (is.numeric(na.treat)) object[[task]][,value][is.na(object[[task]][,value])]=na.treat
         else if (is.function(na.treat)) object[[task]][,value][is.na(object[[task]][,value])]=na.treat(object[[task]][,value][is.na(object[[task]][,value])])
         else if (na.treat=="na.rm") object[[task]]=object[[task]][!is.na(object[[task]][,value]),]
       }
     }
   }
 
   attr(object,"algorithm")=algorithm
   attr(object,"value")=value
   attr(object,"case")=case
   attr(object,"annotator")=annotator
   attr(object,"by")=by
   attr(object,"largeBetter")=!smallBetter
   attr(object,"check")=check
   class(object)=c("challenge", class(object))
   object
 }