New York/York/Boston => New York/Boston

##</code><code>##

    1 while s{
      (/|^)   # add: capture slash or start of string: $1
      ([^/]+)      # non-slashes (the city): \2
      /            # a /
      (?=          # look ahead for:...
        (?:        # group (not capture)
          [^/]* /  # city and /
        )*?        # 0 or more times, non-greedily
        \2         # is the city here again?
        (?: /|$ )  # a / or end-of-string
      )
    }
    {$1}gx;  # add: replacement

##</code><code>##

 1 while s#(/|^)((?>[^/]*))/(?=(?:.*?/)?\2(?:/|$))#$1#g;