What would happen if I accidentally put the same address on both boxes of the address line? Would my package get delivered?

Your package will still be delivered as long as you put a valid city, state, and zip code. If you left off an apartment number or other information, you might have trouble receiving your package. If your address was something like this, you'll be fine:

Your name

123 pizza road

123 pizza road

Your city, Your state, your zip code

No problem, except that if for some reason it is undeliverable, you won't get it back. Not only that, but the post office may not accept it due to security issues (legitimate shipper ID required). If it has the same address for the sender and the shipping address, when you take it to the post office, they will ask for an ID with your address on it. If it is the same address, they will let you do it, if not, then they won't. Senders of packages have to be ID'd for security reasons. If you did it by accident, the package will be delayed while it undergoes security processing - as potentially dangerous.

so long as the rest of the address is there (city, ST, zip), shouldn't be a problem