$cities = "Here/There/Everywhere/There/Again/Here/Too";

$cities =~ s{
  ([^/]+)      # non-slashes (the city): \1
  /            # a /
  (?=          # look ahead for:...
    (?:        # group (not capture)
      [^/]* /  # city and /
    )*?        # 0 or more times, non-greedily
    \1         # is the city here again?
    (?: /|$ )  # a / or end-of-string
  )
}
{}gx;