I have these dnd books dnd handbook and xanthus guide and I'm getting the monster manual for christmas but i want to be able to use them on dnd beyond but I don't want to but them on this site for a second time is there a way for me to use them on dnd beyond without buying them for a second time?    

Unfortunately, no. There isn't a way to activate the content on DDB that you've purchased/been gifted through a physical book. 

ok bummer thanks for letting me know have a nice day

Yeah, 2 different entities selling the same content. DND beyond needs to pay their staff too.

fair enough