R 如何删除列行中的重复项？

我有一个数据帧，df：

a <- c("ID1","ID2","ID3")
b <- c("happy;sad","happy;happy","happy;sad;sad;sad")
c <- c("I","II","I")
df <- data.frame(a,b,c)

    a                 b  c
1 ID1         happy;sad  I
2 ID2       happy;happy II
3 ID3 happy;sad;sad;sad  I

---

拆分、唯一，然后重新粘贴：

df$b <- sapply(as.character(df$b), function(i){
  paste(unique(unlist(strsplit(i, ";"))), collapse = ";")
})
#     a         b  c
# 1 ID1 happy;sad  I
# 2 ID2     happy II
# 3 ID3 happy;sad  I

df$b
df$b <- sapply(as.character(df$b), function(i){
  paste(unique(unlist(strsplit(i, ";"))), collapse = ";")
})
#     a         b  c
# 1 ID1 happy;sad  I
# 2 ID2     happy II
# 3 ID3 happy;sad  I