以 iris dataset 為例，依照 Species 分組列出不重複的 Petal.Width，並黏合字串

library(tidyverse)
library(data.table)

mydata <- iris %>% data.table()
dcast(mydata, Species ~ "Petal.Width", value.var="Petal.Width", fun.aggregate=function(x) paste(x %>% unique(), collapse = ", "))

##       Species                                                        .
## 1:     setosa                             0.2, 0.4, 0.3, 0.1, 0.5, 0.6
## 2: versicolor                1.4, 1.5, 1.3, 1.6, 1, 1.1, 1.8, 1.2, 1.7
## 3:  virginica 2.5, 1.9, 2.1, 1.8, 2.2, 1.7, 2, 2.4, 2.3, 1.5, 1.6, 1.4