Could this be related to React's strict mode in development?

Yes this is what happens when you have your App wrapped in <React.StrictMode>. Most likely in the Index.js

Here is a link explaining more : https://stackoverflow.com/a/61897567/15474532

How can I ensure the useEffect runs only once as intended? Any insights or suggestions would be greatly appreciated!

Remove strict mode. There are some reasons why you would want to keep it. I personally remove it