Generate random code in R across samples for a model and have the option to add extra non duplicate codes if required

I have 20 models (1:20) and 2 samples per model. I want to write a function in R that will take Model# and Sample# as input and output 40 unique random codes across samples for a Model and print the output like this. In addition, I would also like to have the option to add extra columns(if needed) of non repeated random codes like this and extra model numbers.

So far I have this code which gives me this output

food_safety_lot_codes <- function(random_seed = NULL,
                              number_of_cus,
                              number_of_samples,
                              file = NULL){
library(tidyverse)

if(!is.null(random_seed)){
    
    set.seed(random_seed)
    
  }
else{

random_seed <- round(runif(1) * 1000000, 0)
print(paste("Your unique starting seed:", random_seed))

set.seed(random_seed)

}

if((number_of_cus*number_of_samples) <= 899){

digits <- 3
digit_codes <- 100:999

}
else if((number_of_cus*number_of_samples) > 899 &
      (number_of_cus*number_of_samples) <= 8999){

digits <- 4
digit_codes <- 1000:9999

}
else if((number_of_cus*number_of_samples) > 8999 &
      (number_of_cus*number_of_samples) <= 89999){

digits <- 5
digit_codes <- 10000:99999

}
else{

digits <- 0
print("You have requested too many codes.")

}
if(digits != 0){

random_order <- runif(length(digit_codes))

codes.df <- as.data.frame(cbind(digit_codes, random_order)) %>%
  arrange(random_order)
codes.df <- codes.df[1:(number_of_cus*number_of_samples),]

CU <- rep(1:number_of_cus, each = number_of_samples)
Sample <- rep(1:number_of_samples, number_of_cus)

codes.df$CU <- CU
codes.df$Sample <- Sample

codes.df <- codes.df %>%
  rename(`Random Codes` = digit_codes) %>%
  select(CU, `Random Codes`, Sample)
if(!is.null(file)){
  
  filename <- file
  
}
else{
  
  filename <- paste("labels (", digits,
                    " digits).csv",
                    sep = "")
  
}
write.csv(codes.df,
          filename,
          row.names = FALSE)

print(paste("There is now a .csv file in this directory:",
            getwd()))

 }

}

How can I do the additional part? I would highly appreciate any help. Thanks!