How can I generate the number of sets of segments containing the elements 1 to n form a polygon with n sides? (elements can be repeated)

Question

Let\'s say for example n = 3, the sets of segments would be:

(1,1,1)(1,1,1)

(1,2,2)(1,2,2) which is the same as (2,1,2)(2,1,2) which is the same as (2,2,1)(2,2,1)<