I have a simple apex method that generates a short ID and a trigger that attaches that ID to a custom text field on my Account object. 

In the configuration for this custom field, I specified that this field should be unique. 

There is a very slight chance that the method that creates my ID will create a duplicate. So, I'm looking for a way that my trigger can simply try again when Salesforce detects that the ID I just tried to put into my record is a duplicate. EventBus.RetryableException (https://developer.salesforce.com/docs/atlas.en-us.platform_events.meta/platform_events/platform_events_subscribe_apex_refire.htm) seems like the right path to take, but I'm having trouble either getting it to work properly or creating a test that can validate whether it is working. 

Method: 
 

public class GeneratePlanID {
    public static String generateRandomID(Integer len) {
        final String chars = 'ABCDEFGHJKMNPQRSTUVWXYZ23456789';
        String randStr = '';
        while (randStr.length() < len) {
           Integer idx = Math.mod(Math.abs(Crypto.getRandomInteger()), chars.length());
           randStr += chars.substring(idx, idx+1);
        }
        return randStr;
    }
}

trigger PlanIDTrigger on Account (before insert, before update) {
    try {
        for (Account a : Trigger.New) {
            if (Trigger.isInsert || a.PlanID__c == null) {
                String newID = GeneratePlanID.generateRandomID(4);
                a.PlanID__c = newID;
            }
        }
    } catch (Exception e) {
        if (EventBus.TriggerContext.currentContext().retries < 3) {
            throw new EventBus.RetryableException('Exception found retrying plan ID assignment', e);
        } else {
            System.debug('PlanID assignment reached max retries');
        }
    }
}

@isTest
public class TestPlanIDTrigger {
    @isTest static void TestCreateNewAccount() {
        Account acct = new Account(Name='Test Account');
        
        Test.startTest();
        Database.SaveResult result = Database.insert(acct, false);
        Test.stopTest();
        
        System.assert(result.isSuccess());
        
        Account resultPlanID = [SELECT PlanID__c FROM Account WHERE ID = :result.getId()];
        System.assert(resultPlanID != null);
    }
    
    @isTest static void TestDuplicateAccountPlanID() {        
        
        Test.startTest();
        Account[] accts = new List<Account>{
            new Account(Name='Test Account 1', PlanID__c='ABCD'),
            new Account(Name='Test Account 2', PlanID__c='ABCD')
        };
        Database.SaveResult[] result = Database.insert(accts, false);
        Test.stopTest();
        
        for (Database.SaveResult sr : result) {            
            if (sr.isSuccess()) {
                System.debug('Successful insert of PlanID');
                System.assert(sr.isSuccess());
            } else {
                for (Database.Error err : sr.getErrors()) {
                    System.debug('Failed insert of PlanID');
                    System.debug(err.getStatusCode() + ': ' + err.getMessage());
                }
                System.assert(!sr.isSuccess());
                System.assert(sr.getErrors().size() > 0);
            }
        }        
    }
}