Make it so the user can only vote one time for each question

src/services/twitch_handler.ts

@@ -73,7 +73,11 @@ export const run_twitch = (): void => {
 
 			// Check if the user is voting
 			let match = message.match(/\((?<choice>[A-F])\i)/)
-			if (match && question_index != -1) {
+			if (
+				match // choice found in message
+				&& users[context.username] == null // user not voted yet
+				&& question_index != -1 // quiz started
+			) {
 				let choice: string = (match as any).choice;
 
 				// Update the user's answer