Hold a total-market fund, an S&P 500 fund, and a technology fund, and you do not hold three diversified things. You hold the same forty companies at three different weights, plus a large deliberate tilt you never chose in those words.
The vocabulary problem
The industry has a term for this — active weight — and using it is how the conversation ends. So we show the overlap instead: here is what you own once, here is what you own three times, here is the share of your portfolio sitting in one sector.
No recommendation follows. Concentration is not a mistake; an unnoticed concentration is just a decision someone else made for you.