I've blogged about word echoes before. They used to be a particular problem for me, one I like to think I've now mostly got the better of. But when I've nearly finished writing a book, as well as reading it aloud, I put the text chapter by chapter through editing software to catch repeated words.
Initially I used Autocrit, then it got expensive. I switched to ProWritingAid, which was free, but you now have to pay if you want to analyze more than 500 words. I don't object to paying for a service, and it's easy to scout around and find 25% off vouchers, but ProWritingAid offers a wealth of features I wouldn't use. I don't need help with my grammar, style or readability. Their idea of a 'sticky' sentence is not mine. I just want to find repeated words...
The answer is Repetition Detector. It's simple and does one job really well, highlighting repeated words in two categories, close and distant repetitions, and you choose the definition of close and distant. You can tell it to skip words like he, she, it etc.. It's free for 30 days, then an incredibly bargain $7.40 for life.