They are two separate phrases, with very similar meanings.
I found some of those posts confusing but here are the definitions and examples given in the Collins dictionary:
knock-on effect (British): something that results inevitably but indirectly from another event or circumstance.
The knock-on effect is seen in our neighbourhoods and communities.
The cut in new car prices has had a knock-on effect on the price of used cars.
Domino effect SINGULAR NOUN
If one event causes another similar event, which in turn causes another event, and so on, you can refer to this as a domino effect. The domino effect if one train is cancelled is enormous.
If something causes something else to happen, then you�d say it has had a knock-on effect.
If something causes something else to cause something to happen. Which in turn, causes something else to happen. Which in turn.. then you�d call it a Domino effect. Just like a series of dominoes falling.
I hope that helps!