Thanks for your help! It works perfectly now.
The issue is happening on a single feed. The RSS feed is: https://pubmed.ncbi.nlm.nih.gov/rss/search/1T5FW5K6kI7_ib25dnekyMtEXpGBLaOr06kN1qxSU80qPUWQcW/?limit=1000&utm_campaign=pubmed-2&fc=20230607213154
I previously manually deleted all the duplicates, then somehow the duplicates are regenerated over time.
Thanks @mikeyboo, I am using the free WP RSS Aggregator. I have made a custom template with my RSS feed, and used the code [wp-rss-aggregator template="Custom"] to bring up the feed on a page. My debug log and system info are here.
