Add validation for duplicate team entries

- Check for duplicate members and alumni
- Create separate validation function as suggested
- Fixes #1918

Author: vivekjami

src/validate.rs

@@ -28,6 +28,7 @@ static CHECKS: &[Check<fn(&Data, &mut Vec<String>)>] = checks![
     validate_subteam_of,
     validate_team_leads,
     validate_team_members,
+    validate_duplicate_team_entries,
     validate_alumni,
     validate_archived_teams,
     validate_inactive_members,
@@ -236,6 +237,37 @@ fn validate_team_members(data: &Data, errors: &mut Vec<String>) {
     });
 }
 
+/// Ensure no duplicate entries in team members or alumni
+fn validate_duplicate_team_entries(data: &Data, errors: &mut Vec<String>) {
+    wrapper(data.teams(), errors, |team, errors| {
+        // Check for duplicate members
+        let mut seen_members = HashSet::new();
+        for member in team.explicit_members() {
+            if !seen_members.insert(&member.github) {
+                errors.push(format!(
+                    "team `{}` has duplicate members: {}",
+                    team.name(),
+                    member.github
+                ));
+            }
+        }
+
+        // Check for duplicate alumni
+        let mut seen_alumni = HashSet::new();
+        for alum in team.explicit_alumni() {
+            if !seen_alumni.insert(&alum.github) {
+                errors.push(format!(
+                    "team `{}` has duplicate alumni: {}",
+                    team.name(),
+                    alum.github
+                ));
+            }
+        }
+
+        Ok(())
+    });
+}
+
 /// Alumni team must consist only of automatically populated alumni from the other teams
 fn validate_alumni(data: &Data, errors: &mut Vec<String>) {
     let Some(alumni_team) = data.team("alumni") else {