At what point does owning SPY + QQQ + a growth ETF stop being diversification and start being the same bet in different wrappers?

I’ve been thinking about this lately and I’m curious how others look at it. Let’s say someone owns SPY, QQQ and VUG. On paper it looks diversified because they’re 3 different ETFs but when you look at what’s actually inside them, there’s quite a bit of overlap and they’re all pretty exposed to large US growth/tech names. So at what point are you not really “diversifying” anymore and just adding more weight to the same type of companies? I’m not saying that’s necessarily bad. If someone intentionally wants a growth tilt, that’s completely different. What I’m more curious about is people who think adding another ETF automatically means more diversification. Would you mainly look at holdings overlap for this? Or is correlation during market selloffs more important? For example, I’d probably want to compare things like max drawdown, volatility, how long it took to recover, and whether all 3 funds basically fell together during bad periods. Long-term return by itself doesn’t really answer the question because a portfolio can do really well over 10 years and still be taking a lot more concentrated risk than you realize. How do you guys usually judge this? Holdings overlap? Sector exposure? Correlation? Drawdowns? Or something else?