Good expository writing is concise—all the necessary words, but no more. Redundancy is extra words that you don’t need. Those extra words say what you already said.
Here are a couple sentences from a NASA press release. I’m a bit ashamed to reveal the source because the press release contains a bunch of bad writing (including carelessness: “…including released of water vapor.”)
These two sentences each contain a redundancy:
The ongoing examination of Bennu – and its sample that will eventually be returned to Earth – could potentially shed light on why this intriguing phenomenon is occurring.
Many of the ejected particles are small enough to be collected by the spacecraft’s sampling mechanism, meaning that the returned sample may possibly contain some material that was ejected and returned to Bennu’s surface.
Here’s the rule, dating back at least as far as Joel Chandler Harris:
If you can leave a word out, leave it out.
PS—I ran into a comic that makes the point, too: