Can you really househack every year?

[Alec Jacobs]

I hear a lot of people say something like "just buy a duplex every year" when it comes to househacking but is that actually possible?

This is hearsay but I heard a story about someone saying they tried to buy a 2nd househack after their first one but they couldn't because they needed "20% equity" in their first home. 

Is this an actual concern or is it just not true. And if it is a problem that you are most likely to face why haven't more people talked about it?