Code fired twice? How could this happen?

Hey guys,
I’m having the following async cronjob which executes every hour:"TRANSACTION JOB STARTED");
    let transactions = new TransactionImport();

    console.log("FETCHED NEW TRANSACTIONS: "+ new Date().toString());

    // After fetching, we start optimizing...

    let optimizer = new Optimizer(moment(new Date()).subtract(serverConfig.CHECK_PERIOD, 'days').toDate());

    console.log("FINISHED CRONJOB: "+ new Date().toString());

After checking todays logs, I saw the following interesting thing:

FETCHED NEW TRANSACTIONS: Mon Apr 23 2018 18:01:05 GMT+0200 (CEST)
Started opt. calculation on instance 0
FINISHED CRONJOB: Mon Apr 23 2018 18:06:53 GMT+0200 (CEST)
FINISHED CRONJOB: Mon Apr 23 2018 18:06:53 GMT+0200 (CEST)

The FINISHED CRONJOB message fired twice, but the cronjob wasn’t executed twice, because all other messages are only posted once within the logs. So I’m wondering how could this happen? Any idea what did go wrong? I’ve checked the logs because the job didn’t add all new values to the database. This never happened before.