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# Count of each value of "PERSOONSNUMMER" in data
count(data, vars = "PERSOONSNUMMER")

# merge with dataset
data_new <- merge(data, new, by = "PERSOONSNUMMER") 

# Example

# Create variable of total number of uitkeringen per person
d <- count(uitk, vars = "PERSOONSNUMMER")
uitk <- merge(uitk, d, by = "PERSOONSNUMMER") 
colnames(uitk)[colnames(uitk)=="freq"] <- "Total_Nr_Uitk"