don't invite an already invited email (#8688)

* don't invite an already invited email

* add email-invited translation string

public/language/en-GB/error.json

@@ -31,6 +31,7 @@
 
 	"username-taken": "Username taken",
 	"email-taken": "Email taken",
+	"email-invited": "Email was already invited",
 	"email-not-confirmed": "You are unable to post until your email is confirmed, please click here to confirm your email.",
 	"email-not-confirmed-chat": "You are unable to chat until your email is confirmed, please click here to confirm your email.",
 	"email-not-confirmed-email-sent": "Your email has not been confirmed yet, please check your inbox for the confirmation email. You won't be able to post or chat until your email is confirmed.",

src/user/invite.js

@@ -43,10 +43,16 @@ module.exports = function (User) {
 		const expireDays = meta.config.inviteExpiration;
 		const expireIn = expireDays * 86400000;
 
-		const exists = await User.getUidByEmail(email);
-		if (exists) {
+		const email_exists = await User.getUidByEmail(email);
+		if (email_exists) {
 			throw new Error('[[error:email-taken]]');
 		}
+
+		const invitation_exists = await db.exists('invitation:email:' + email);
+		if (invitation_exists) {
+			throw new Error('[[error:email-invited]]');
+		}
+
 		await db.setAdd('invitation:uid:' + uid, email);
 		await db.setAdd('invitation:uids', uid);
 		await db.set('invitation:email:' + email, token);