fix(error): Don't duplicate args in usage

[epage]

Gave up trying to decipher the existing logic for safe ways to
de-duplicate manually and switched to an IndexSet to enforce only one
of each argument exists.

Fixes #3556

[leolovenet]

By the way, there are two more required checks here, before calling create_usage_with_title, But it doesn't matter if you keep them the same

clap/src/parse/validator.rs

Line 130 in dcda237

!(a.is_hide_set() || self.required.contains(&a.id))

clap/src/parse/parser.rs

Line 1474 in dcda237

.map_or(true, |a| !(required.contains(&a.id) || a.is_hide_set()))

[epage]

Thanks! I missed those