Example : {1,2,2,3,3}

the answer here is 5 :

{1,2,2} {1,2,3} {1,3,3} {2,2,3} {2,3,3}

I want a formula to calculate this.

I searched through Internet but I did not find a clear answer.

I am not good at math so I want a simple explination If that is possible. Thank you.