Is it only me or it's impossible to recognize in normal speech if it's een or de in this particulat sentence? The "n" is hardly pronounced (if at all, i couldnt hear that). As een comes right after heeft, you hear something like "hevte" which can as well be heeft een and heeft de.
You are mixing up 1 with een But duo is to blame for that. In the first lessons frustratingly all the een were pronounced as één. So as the number one.
As a native speaker (on here trying to help people with questions) it was very hard to keep hearing that each lesson and still continue. So double frustrating, each time you think "no its not 1 it's een!" (But the phone doesn't listen) But also the thought that everybody will now learn the wrong thing is painfull.
My only hope was well if they ever get to the netherlands they will soon realise the error. Since een is in nearly every sentence.
But yea big mess up from duo and odd that it is still not fixed. 6 years after the start of the course..