Usually it means two or more people are doing the same job, and it's unnecessary, because tasks haven't been agreed on in advance.
For example, if we're planning a big event, we need to write and send invitations, book a venue, organise food, organise music etc. If we don't talk about it in advance, maybe we'll both call and book different bands. We'd then have to cancel one of them. So basically, we've done way more work than necessary.