search for: dff3

...)), yy=rnorm(11)) # note that the two groups "gr1" and "gr2" have defined levels which not occur in the data. dff2 <- split(dff, list(dff$gr1, dff$gr2), drop=TRUE) # I dont want empty objects, so I use drop=TRUE # now I want to unsplit it, and expect the following to work: dff3 <- unsplit(dff2, list(dff$gr1, dff$gr2), drop=TRUE) Error in `row.names<-.data.frame`(`*tmp*`, value = c("1", "11", "3", "11", : duplicate 'row.names' are not allowed In addition: Warning message: non-unique values when setting 'row.names...