As per my understanding , you will be calling the same dropdown property in all the rows of a repeat grid and the selected value in first row should not appear in the next row and it goes on.Correct me if I am wrong
This may lead to performance issue, since for every row dropdown property needs to be iterated and the selected value has to be skipped.I suppose we can achieve this by calling a activity but not sure.
please share below information for more clarity
1) Pega Version
2) How you are sourcing the dropdown
3) What If the numbers of rows in the repeat grid are greater than the values in the dropdown ?