I have had implantation bleeding and also brown spotting all the way, it is true that is not normal but it may be nothing too. So they told me to calm down and just act normally and wait for the first trimester to be over.
I had an u/s last week and saw a heartbeat, so we are praying for the baby to stay with us!
About the hormone levels, it's always good is they double, it's actually weird that they told you otherwise. Where I live they never measure hormone levels in the blood, they just make you wait for another week and do another u/s to see if the yolk sac is growing, etc.
Hopefully everything goes alright with your pregnancy, I know a lot of women that had bleeding at some point and didn't mc.