Do I have to remove exsiting counter before installing granite??

Make sure everything is disconnected to it like your dish washer. Remove backsplash. Look underneath to find where it us screwed down. Remove screws and pull out.

Yes, old counter needs to be removed.

Youcan place it on top of old countertops, but the cabinet has to be strong enough for the weight..my sister had to have her cabinets reinforced before putting the granite on...

Usually there is a plywood base under granite. So yes you would need to remove your existing countertops.

I had a "granite overlay" installed and it was perfect. It is 3/8 inch thick and they put it right over the top of my formica counter top. It is actually a lot stronger than the granite as it has the solid base underneath. AND, I didn't have to take anything out of the cabinets. They cut the hole for the sink the day they installed the top and it didn't take very long at all, maybe a couple of hours, (if that). No one can tell it from the usual granite top as it looks the same.

most people would because of the extra height and weight

Yes the fabricator will measure for plywood, you do not want to make the counter too top heavy, you will ruin the cabinets underneath. You can leave your backsplash if you are not replacing that.