Why are there two lines?

Chanterelle
So I took a hpt yesterday around 5:30pm and during the time window it was negative so I'm going to go with that but I looked at it today and there were two faint lines...I inverted the picture so I could see better...I know the test is invalid if you look after the time period but I was just wondering why does two lines show up and if they were an Evap and indent so I can know what to look for in future tests?