How was a user able to post a question with a title identical to an existing?

The following question was asked 5 days ago:Object Reference not set to an instance of an object

How was the author able to post the question when there are already 58 old other questions with the same exact title?

I've checked that:

• The post was not migrated
• The post was not deleted then undeleted
• Its title does not contain invisible characters

What I did not check, because unlikely:

• The author is probably not an employee of Stack Overflow, Inc

(In case you're skeptical about those 58 identical titles, here are some random picks: 1, 2, 3, 4, 5, ...)