Assuming you enter the formula starting in say, K7, try:

=IFERROR(INDEX($E$8:$E$28,MATCH(1, IF($E$8:$E$28<>"(Must Choose)",IF($E$8:$E$28<>"",IF(ISNA(MATCH($E$8:$E$2 8,K$6:K6,0)),1))),0)),"")

change the range K$6:K6 to match cell above where you enter formula.

Then confirm the formula with CTRL+SHIFT+ENTER to make it an Array formula, then copy it down.