Well, we have to understand the fact that if we allocate say.... 10% of our portfolio to tech stocks, purchasing another 10% in a tech ETF is going to bloat our exposure to 20%.
However, say your portfolio contains 10% Shopify,Amazon and Facebook and you are looking at an ETF that isn't specifically focused on tech that has a 6% total allocation to the three (I'm really just spitballing figures here) that really wouldn't concern me as 94% of the ETF is elsewhere.
I hope this makes sense. Let me know if it doesn't.