Checklist

• [y] Are you running the latest v2 release? The list of releases is here.
• [y] Did you check the manual for your release? The v2 manual is here
• [y] Did you perform a search about this feature? Here's the Github guide about searching.

What problem does this solve?

Sometimes users can accidently duplicate string type flags and then the latest value we be taken. It is not clear and bad for usability.

Solution description

Add an option called Duplicated true\false to flag options. Example:

flagFoo = &cli.StringFlag{
	Name: "foo",
	Duplicated: false,
}

A detailed description of what you want to happen.

if Duplicated set to false command should fail with error
"flag foo can not be duplicated"

Describe alternatives you've considered

For now, as a workaround, we're using StringSliceType for flags we don't want to be duplicated and check that slice have only one element in it.

https://github.com/Venafi/vcert/blob/8e1dc13c0afdd615d7f27b80190ddf3a1d07e848/cmd/vcert/commands.go#L105-L120