They are'nt really all that similar because Amity are peaceful people, while Abnegation are selfless. So, Amity leaders work for peace, while Abnegation leaders give to others. And your right, Duantless have to be selfless as well, as explained by Tris and Four in the first book. A lot of the factions are the same, sorta. But each of them is differnt. Dauntless are brave; Erudite are intelligent; Amity are peaceful; Candor are honest; and Abnegation are selfless. So there are reasons for all of the factions. And selfless are not slaves, just generous people.
I can see where this question is coming from, because the two A-factions are both about "other people." But the emphases are different. Amity wants everyone to get along; Abnegation wants to be a helper instead of a burden. So you could technically be a hermit and be in Abnegation (if they had hermits in Divergent); and you could be an entertainer and be in Amity. But I'd have a hard time seeing either of those in the opposite faction...say, an Abnegation leading a singalong or an Amity silently withdrawing so nobody thinks they're in the way. It just doesn't fit with the overall philosophy of those factions.
An Amity doesn't have to give to others. They could just politely say they won't. To them it is more important to make someone feel better on the inside than it is to do someone a practical favor, An Abnegation would do anything for you, but they could be acting...for the lack of a better expression, stiff the whole time.
Imagine asking peole for a favor. An Amity may refuse in a super kind way, and an Abnegation will do it, albeit possibly not making the most pleasant company. If you've ever met someone who agrees to help you acting friendly...I guess they are Divergent.
Hey, this is fun. This is why I like Psychology. I'd like more questions like this.