Your assumptions were correct, so I think you already got it.
You are in fact correct that Construct 3 automatically prevents re-includes, so the first include of that event sheet will always be chosen. (My understanding, from when I looked this up.)
Now for an architecturally sound perspective, having all your includes into the event sheet of that layout is important. From my understanding, you avoid event order issues this way. And, you will make your project modular and organized.
Most software development paradigm follows this structure too when importing libraries, their version of including event sheets. So, I think it's a safe guarantee that this is a right way.