Using LockService to avoid concurrent executions, specially relevant when invoking GAS code from buttons in a spreadsheet.

ButtonLock.gs

/**
 * Simple demonstration of LockService...
 * https://developers.google.com/apps-script/reference/lock
 * ...to prevent concurrent executions of a GAS function, 
 * for example when invoked by a button in a Spreadsheet (rather likely).
 * Demo: https://docs.google.com/spreadsheets/d/1n_7nZIi5QtN9URzsKdFW6WtSxPM9uUK3QhQSbzbltHg
 *  
 * @OnlyCurrentDoc
 */

function myExclusiveFunction() {

  const t1 = new Date();
  const ss = SpreadsheetApp.getActiveSpreadsheet();
  const exclusiveMode = ss.getActiveSheet().getRange('A4').getValue();
  const lock = LockService.getDocumentLock();

  if (!exclusiveMode || lock.tryLock(0)) {

    // Lock acquired or not in exclusive mode
    ss.appendRow([t1.toLocaleTimeString(), '', '▶️ Start']);

    // Task to perform: just go idle for 3 seconds
    Utilities.sleep(3000);
    
    ss.appendRow([new Date().toLocaleTimeString(), '', `⏹️ [${t1.toLocaleTimeString()}] Finished`]);

    if (exclusiveMode) {
      lock.releaseLock();
    }
  
  } else {
    ss.appendRow([t1.toLocaleTimeString(), '', '🛑 Execution aborted, lock active']);
  }
}