I got lucky and got some IA expansions on closeout and I'm going to sell off what I don't need. I'm getting into skirmish though, so I'm not sure if duplicates of some of these packs are going to do me any good. I'm still pretty new to the game.
So, I've got at least one duplicate of each of the following, will I wish I didn't sell these as I build lists later? Are there any I'd want more than two packs of?
(xpost /r/ImperialAssaultTMG)
The second core set is definitely worth it, and I'd keep at least 2 Rebel Sabs. Otherwise, sell any doubles of unique figures.
unless you have a friend who may be interested as well. You'll get way more enjoyment out them if you can share them with a friend and get them interested than whatever few dollars you may gain. Most of the duplicates I have purchased, were to share with family members and now I have more people to play with. 
Yeah you don't need dupes of Unique characters but multiple sabs is good.
Having two cores can help, but I only have one and have never been wishing I had two. You can get by without two cores especially as more and more expansions come out. But if you want to be as serious as possible you'd better keep it.
unless you have a friend who may be interested as well. You'll get way more enjoyment out them if you can share them with a friend and get them interested than whatever few dollars you may gain. Most of the duplicates I have purchased, were to share with family members and now I have more people to play with.
I do play the RPG as well, Those duplicates could be characters. Good point.
Sell it all.
If you want to really go in with skirmish then you'll really want to keep the Saboteurs. More than 2 sets is probably not necessary, but IMHO you'll almost surely want 2.
I have 2 Core Sets and I am happy with the purchase. However, I have 3 boys who all play (and attend tournaments) with me so having the second set is great. You'll probably be fine with just 1 Core, but there's so much value to that box that it's hard to part with...especially if you were able to get it for a good price.
And yeah...Uniques are only worth having multiples of if you share your collection (as I do with my boys).
Personally I'd sell all the uniques and keep the duplicates of the other bits as others have said.
The most useful ones are probably having 2 reg/elite Nexu (Promo available), 2 elite Probes, 2 Regular Royal Guard (Promo available), 2 Regular Trandoshans. There's probably some bits that I've missed there but that's the most useful I can think of off the top of my head.
Depends though, if you're not likely to attend tournaments I'm not sure it's worth it, I like to compete in tourney's locally so need those cards