I'm sure you have all heard the phrase, "History repeats itself" more than once. I'm of the mind that history does, indeed repeat itself until we, as a whole, the human race, learn from our mistakes.
What does that have to do with writing? Well, if you are writing a series of stories, you will find you have to repeat yourself. Mainly to keep the reader focused on what is happening, overall, but also to remind them what happened in previous books. You give them a down and dirty refresher course, especially if they've put down the series for a bit and are picking it back up.
But those aren't the only reasons to repeat. Some story lines have repeating, or similar plots, for a lot of reasons. One being, the plot works. It might get boring and repetitive, but it is mirroring real life situations.
Let's be honest. The battle between Good vs. Evil doesn't end after one battle. Someone else will pick up the mantle on either side and the struggle will continue. It's never-ending. A lot of the time, it's a repetition of battles of past days.
A lot of people don't like repetition. Some do. I'm not a huge fan of it, quite frankly, I get irritated with an author who reminds me of what's going on, every other page. I started to read a series I had been collecting for a while, now. One I was really looking forward to reading. Each book would become available for free on my Kindle and I'd snatch the installment and hoard it away like a dragon clutching to his treasure. Finally, I had all the books in the series, and still, I put off reading it, afraid of disappointment.
I began reading the series and got to the third book before I was just fed up. I was being reminded constantly of what was happening, which dragged the movement of the story down to a slow crawl, yet everything that happened to the character, happened in a space of three days. A LOT happened! Enough happened to the Main Character, that this series could have taken each event and had a whole book for each event. But that's another topic for another time.
The repetition in the story, the reminders of what was happening, started to come across as nagging. ("Don't forget to take out the trash!")
Remind your readers, if you are writing a series, in the beginning of the book and move on. If something pops up that requires explanation of an older story, make it into dialogue to keep things rolling. As for repetitive plots? Write and repeat until your characters learn the lesson, hear the message of what their universe (you, the author) are trying to tell them.