v1<-c(1,1,1,1,1,2,2,2,3,3,3,3,3,3,3)
v2<-c("Jan","Jan","Jan","Feb","Feb","Jan","Jan","Feb","Jan","Jan","Feb","Feb","Feb","Feb","Feb")
v3<-c("A1","E1","F1","B1","A1","E1","B1","C1","B1","D1","E1","A1","B1","C1","F1")
dt <- data.table(emp_id=v1,month=v2,work=v3)

temp1 <- dt[,.(list(work)),.(emp_id,month)]
head(temp1)

temp2 <- temp1[,.(list(V1)),.(emp_id)]
head(temp2)

temp2[,V1 := lapply(V1, unlist, use.names = F)]
dt <- setnames(temp2,"V1","Events")