Can I claim the American Opportunity tax credit for a second undergrad degree? I graduated with a Bachelor's degree in 2000, and going for associate's in 2014.

No. You have completed four years of post secondary education. You may be eligible for the less generous Lifetime Lerning credit or tutiion and Fees deduction 

What if you only claimed the AOC once during your first degree? Or if you never claimed the AOC before. Are you able to claim the rest in your second degree?

No. in addition the 4 times rule, there is a another rule that say you must not have completed 4 years of post secondary education.