library(data.table)
#Create the fictional data up to the current choice.
choices<-c(1:100) #vector of possible choices
people<-data.frame(ID=1:10)
setDT(people,key="ID")
people[,"current_choice":=sample(choices,1),by="ID"] #what the person uses now
people[,"chosen":=sample(choices,1),by="ID"] #what the person actually picked at t_2



#expand the dataset to be 30 rows per person and create a choice ID
people<-people[rep(1:.N,30),]
setDT(people,key="ID")
people[,"choice_id":=seq_len(.N), by="ID"]

#The current choice at t_1 needs to be in the choice set
people[1,"choice_set":=current_choice,by="ID"]

#The actual choice needs to be in the choice set
people[choice_id==2&current_choice!=chosen,"choice_set":= chosen,by="ID"]

#I want the remaining choices to be sampled from the vector of choices, but here is where I'm stuck
people[is.na(choice_set),"choice_set":=sample(choices,1),by="ID]