trigger ValidateAccount on Account (before insert) {
    Set<String> accountNames = new Set<String>();

    for (Account account : Trigger.New)
    {
        if (accountNames.contains(account.Name))
        {
            account.Name.addError('Account name already exists!');
        } 
        else {
            accountNames.add(account.Name);
        }
    }

    List<Account> existingAccounts = [
        SELECT Id, Name 
        FROM Account 
        WHERE Name IN :accountNames
    ];

    for (Account account : Trigger.New) 
    {
        for (Account existingAccount : existingAccounts)
        {
            if (account.Name == existingAccount.Name && account.Id != existingAccount.Id)
            {
                account.Name.addError('Account name already exists!');
            }
        }
    }
}