Here is another way of doing it..
df <- read.table(textConnection("1|a,b,c\n2|a,c\n3|b,d\n4|e,f"), header = F, sep = "|", stringsAsFactors = F)
df
## V1 V2
## 1 1 a,b,c
## 2 2 a,c
## 3 3 b,d
## 4 4 e,f
s <- strsplit(df$V2, split = ",")
data.frame(V1 = rep(df$V1, sapply(s, length)), V2 = unlist(s))
## V1 V2
## 1 1 a
## 2 1 b
## 3 1 c
## 4 2 a
## 5 2 c
## 6 3 b
## 7 3 d
## 8 4 e
## 9 4 f