Added a server side check to see if members who are trying to be invited have already been invited by checking the email. Client handles it as if it was an email error.

api/team.go

@@ -456,6 +456,15 @@ func inviteMembers(c *Context, w http.ResponseWriter, r *http.Request) {
 		user = result.Data.(*model.User)
 	}
 
+	var invNum int64 = 0
+	for _, invite := range invites.Invites {
+		if result := <-Srv.Store.User().GetByEmail(c.Session.TeamId, invite["email"]); result.Err == nil || result.Err.Message != "We couldn't find the existing account" {
+			c.Err = model.NewAppError("invite_members", "This person is already on your team", strconv.FormatInt(invNum, 10))
+			return
+		}
+		invNum++;
+	}
+
 	ia := make([]string, len(invites.Invites))
 	for _, invite := range invites.Invites {
 		ia = append(ia, invite["email"])