Looking on the internet deeply has found these results:

it's common knowledge is the most popular phrase on the web.

it's a common knowledge 1218,000 results on the web	it's common knowledge 1439,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
It doesn't look like this phrase is very popular! The search could not find any examples on the internet.	So it's common knowledge on the street what Gibbons up to? It's common knowledge about you and Elizabeth Tyson. So it's common knowledge that she's his daughter? It's common knowledge you can get anything in the laundry. It's common knowledge that my half It's common knowledge the vic was shot in the face. It's common knowledge that a ninja mansion has traps. Now, it's common knowledge that he's a friend of the cause, but nothing ties him.

it's a common knowledge

1218,000 results on the web

it's common knowledge

1439,000 results on the web

More popular!

It doesn't look like this phrase is very popular! The search could not find any examples on the internet.

So it's common knowledge on the street what Gibbons up to?
It's common knowledge about you and Elizabeth Tyson.
So it's common knowledge that she's his daughter?
It's common knowledge you can get anything in the laundry.
It's common knowledge that my half
It's common knowledge the vic was shot in the face.
It's common knowledge that a ninja mansion has traps.
Now, it's common knowledge that he's a friend of the cause, but nothing ties him.