add checks for missings and duplicates in id variable

[milanwiedemann]

I was thinking of something like this, get vector of all ids and then sum the duplicates and missings, this can then be checked and used for error msg

ids <- sgdata$id
sum_id_duplicates <- base::sum(base::duplicated(stats::na.omit(ids)))
sum_id_missings <- base::sum(base::is.na(ids))