An integer will be added as suffix in x if an identical value is in y.
If a value with suffix is already in y, the function will search for the
highest suffix to avoid duplicated values.
add_suffix(x, y, sep = "_")A character value, either x or x with suffix if already in y.
(character of length 1) The name to be compared.
(character) Existing names, with or without suffixes.
(character of length 1) Symbol to be placed between the original
name and the suffix.