You don't get anything in once or in one go

[Abcd123kkk]

My mother is explaining something to my brother again and again . My brother gets irritated and says:

I get it in once .
Or
I get it in one go.

What should be used in this sentence? "In once or in one go"? Do these sentences sound idiomatic?
Thank you

 

[Copyright]

I got it the first time.

 

[Franco-filly]

"I got it in one!" would sound idiomatic to me, i.e. I understood it the first time you said it.
cross-posted with Copyright - whose version I agree with

 

[Copyright]

"I got it in one!" would sound idiomatic to me, i.e. I understood it the first time you said it.

For BE, I think ... I don't think that would be common in AE.

 

[Abcd123kkk]

And can mom say:
Oh yeah ! As if you get anything in the first time.

 

[Copyright]

Oh, sure ... as if you get anything the first time.