Can only select a drop down option once

Hi

In AA1, put the formula.

=IF(COUNTIF($B$2:$B$22;Z1)>=1;"";ROW())

Copy down.

In AB1:

=IF(ISNUMBER(SMALL(IF($AA$1:$AA$20="";"";ROW($AA$1:$AA$20));ROW(1:1)));INDIRECT("Z"&SMALL(IF($AA$1:$AA$20="";"";ROW($AA$1:$AA$20));ROW(1:1)));"")

Array formula>>CSE

Then

Insert>>Name>>Define, put the formula

=OFFSET($AB$1;0;0;COUNTA($AB$1:$AB$20)-COUNTBLANK($AB$1:$AB$20);1) and give a name. =Teams.

Now In List Validation(column b)>>=Teams

Hope to helps you.