ensure that leaderboard-displayName pair is unique

src/models/leaderboardModel.ts

@@ -13,6 +13,7 @@ const leaderboardEntrySchema = new Schema<ILeaderboardEntryDatabase>(
 );
 
 leaderboardEntrySchema.index({ leaderboard: 1 });
+leaderboardEntrySchema.index({ leaderboard: 1, displayName: 1 }, { unique: true });
 leaderboardEntrySchema.index({ expiry: 1 }, { expireAfterSeconds: 0 }); // With this, MongoDB will automatically delete expired entries.
 
 export const Leaderboard = model<ILeaderboardEntryDatabase>("Leaderboard", leaderboardEntrySchema);