validate that team members don't contain duplicates

[marcoieni]

E.g. rylev was duplicated in

team/teams/mods-venue.toml

Line 9 in 0d40a7b

"rylev",

The validation should be added somewhere around here:

team/src/validate.rs

Line 224 in 0cedf17

fn validate_team_members(data: &Data, errors: &mut Vec<String>) {