What is the difference between one or two V check marks on the vscode?
In particular, what is the meaning of the double v marked?
First of all, that is not a 'v' but, it is a Tick Symbol aka a Check Symbol.
A Single Check represents that Prettier is enabled and working in your current environment and double-check symbol represents that your current document is "fine" according to the rules specified in the Prettier Extension.
Just for future references, refer to this official documentation's article for the meaning of various icons/symbols used across Visual Studio Code.
