QIf I have multiple cards with an "At the start of your end phase" ability triggering, in what order do I resolve them?

AYou can resolve them in any order. When you proceed to your end phase, all "At the start of your end phase" abilities are triggered and become pending. Then, you can choose the order in which you resolve the pending abilities.

QI have 2 copies of this follower on my field at the start of my end phase, and they both have 5 defense. How do I resolve their abilities?

AYou must resolve the "at the start of your end phase" ability of both copies, and both of them end up in the cemetery.

First, at the start of your end phase, the corresponding abilities of both copies trigger and become pending. You then resolve them one by one.

The first copy's ability deals 5 damage to each other follower on the field, bringing the defense of the other copy to 0 and putting it into the cemetery.

Even though the second copy is now in the cemetery, you still resolve its pending ability, dealing 5 damage to each follower on the field, bringing the defense of the first copy to 0 and putting it into the cemetery as well.