Advice please.....

Could I have ovulated twice this month? I ovulated as per the stick on 17th and so I've been testing a few days now with a very faint positive aspects negatives on pregnancy tests. When I put in the pregnancy due date from the first day of my last period it states that my conceived date was 23/24 September. So is that why I am getting negative and faint positive feedback because in theory I'm only 6dpo and not 12 as I first thought? Please help xx