You can try the following (formula 1) assuming there are no Excel version restrictions according to the tags provided in the question:
=LET(pos, A2:A4, accnt, B2:B4, REDUCE({"Account","Position"}, pos, LAMBDA(ac,p,
VSTACK(ac,LET(f,TEXTSPLIT(@FILTER(accnt,pos=p),,","), HSTACK(f, IF(f=f, p)))))))
Here is the output:
![excel output](https://i.stack.imgur.com/RY1dJ.png)