Show options selected in previous question + Additional option

I know how to show in a multi select question only the options selected in a previous multi select question. But I have a different scenario:
Question 1, multi select, 18 options.
Question 2, multi select, should show only options 1-17 selected in question 1, plus option 18 (same as option 18 in Question 1).
I have been playing around with it for a bit but nothing seems to be working.

Thank you very much,

Ludovica, the filtering expression is hence:

(…your original expression…) || (@optioncode==18)

Best, Sergiy