Dropdown List deduction - google-sheets

Question

I have a list of 100 employees and 50 dropdowns. I want to select some of the employees in the dropdowns but I want to prevent the user from choosing the same employee more than once.
I want to deduct the chosen employees from the possible dropdown answers. How can I manage to do so?
I tried to filter out the selected employees but it shows an ugly warning sign for those who were selected
you can view my spreadsheet here:
https://docs.google.com/spreadsheets/d/1bzMPC3-SDOjsgZVXHQgmfBnkiTEsa1Pei7p_DybIebY/edit?usp=sharing

Answer 1

You can use the Simple Trigger onEdit(e) to automatically do the job:
The onEdit(e) trigger runs automatically when a user changes the value
of any cell in a spreadsheet [...]
It's not exactly the same as you had before, because the selected values will still be displayed on the list. They just will be removed if they are repeated.
The only change you should do is to remove the current Data Validation from Column D and set a new one with values from Column A.
function onEdit(event){
if (event.range.getColumn() == 4){ //Run only if the edited cell is from column D
var sheet = SpreadsheetApp.getActiveSheet();
var selection = sheet.getRange("D2:D22").getValues();
var editedRow = event.range.getRow(); //Gets the edited row
for (var i = 0; i < selection.length; i++){
//If some value from D is equal to the edited value but in a different Row (so it ignores the inputted value)
if ((i + 2) != editedRow && selection[i][0] == event.value){
sheet.getRange(event.range.getA1Notation()).setValue(""); //Change the new value to empty
}
}
}
}
The function will run every time you select an item in the column D.
Result:
References:
Range
Sheet
setValue