How do I stop Google AppScripts (Sheets) from creating two protected range entries

I have a Google sheet where I have set a row to become protected when I check a tickbox in Col U. But when I perform this action with the account I setup the code with, it makes two entries of protect ranges.

When I tick the box from another account, I get only 1 entry and it seems to work correctly.

Double entry of range protection with main account

Correct single entry when using a different account

My code is the following. I made it to only protect rows with a warning.

function onEdit() {
  let ss = SpreadsheetApp.getActiveSpreadsheet();
  let sheet = ss.getActiveSheet();
  let cell = sheet.getActiveCell()
  let col = cell.getColumn();//gets column numnber
  let rownum = cell.getRow();//gets row number
 

  if(col==21 && cell.getValue() == true){//if changed to true protects row to end of data
    let protRange = sheet.getRange(rownum,1,1,22).protect().setWarningOnly(true).setDescription(rownum);
    }
  
  else if(col == 21 && cell.getValue() ==false){//if changed to false cycles through all the protected ranges on the sheet and unprotects if the row number is the same
    let protections = sheet.getProtections(SpreadsheetApp.ProtectionType.RANGE)
    for (let i = 0;i<protections.length;i++){
      let proDesc = protections[i].getDescription();
      if(parseInt(proDesc)==rownum){
      protections[i].remove()
      }
    }
  }
}

I changed this part of your code:

if (col == 21 && cell.getValue() == true) {//if changed to true protects row to end of data
  let protRange = sheet.getRange(rownum, 1, 1, 22).protect().setWarningOnly(true).setDescription(rownum);
}

Your code does not check for existing protections. As a result, every time the checkbox is checked, it creates a new protection, which causes the main problem. To handle duplicate protections for the same row, I use the some() method before creating a new one to avoid duplicate entries.

I also converted rownum to a string using toString(), to ensure compatibility with setDescription().

Complete Code:

function onEdit() {
  let ss = SpreadsheetApp.getActiveSpreadsheet();
  let sheet = ss.getActiveSheet();
  let cell = sheet.getActiveCell();
  let col = cell.getColumn();
  let rownum = cell.getRow();

  if (col == 21 && cell.getValue() === true) {
    let protections = sheet.getProtections(SpreadsheetApp.ProtectionType.RANGE);
    let isAlreadyProtected = protections.some(protection => {
      return protection.getRange().getRow() === rownum && protection.getRange().getNumRows() === 1;
    });

    if (!isAlreadyProtected) {
      sheet.getRange(rownum, 1, 1, 22).protect()
        .setWarningOnly(true)
        .setDescription(rownum.toString());
    }
  } else if (col == 21 && cell.getValue() === false) {
    let protections = sheet.getProtections(SpreadsheetApp.ProtectionType.RANGE);
    for (let i = protections.length - 1; i >= 0; i--) {
      if (protections[i].getDescription() === rownum.toString()) {
        protections[i].remove();
      }
    }
  }
}

Sample Output:

Reference:

Class Protection

some()