Just started playing this and I'm wondering who to focus on.
My box.
I invested in ashe because she's been carrying my ass so far, but aside from that I'm pretty clueless about what makes a good unit/party/leader.
And what do I do with duplicate units?
Welcome to the game, first new blood here in a while!
So I looked at your units, you have Heavenly Technician Lid which is a great investment, will make a lot of trials and content easier for you. Ashe is good in Arena even after she's outclassed on your main team. If you max out those 3 Williams, you can Quake chain things into oblivion, which is a great AoE earth move (that rarely has any resists since Earth isn't a common element). Aria is an "ok" healer, but as soon as you get some of the better healers, she'll never be used again. Your biggest need is a tank right now. Warrior of Light is a 4* unit that's really good until you can get a 5* base tank, if you happen to luck into him. You can also get a free 5* base Lightning for completing the beginner quest for her. Pretty outclassed by modern standards, but she's capable of enhancements right now, so she could potentially be a strong damage dealer for you.
So to answer your question, if I were to make a team from your selection in order of importance:
-CG Lid (she's meta right now, great investment for new player)
-Ashe
-William 1
-William 2
-William 3
If you have extra stuff, work on completing the quest to get Lightning (it's pretty easy) and level her up.
Then pick really any high end friend damage unit, and you will annihilate the story up to the end of chapter 1. This team will also be great for you in arena allowing you easy wins on weeks where earth/black magic isn't banned.
Since resources are hard to get at the beginning, I wouldn't waste any on other units until you get a strong healer, tank, or better damage units.
As far as dupes go, you can either hold on to them for chaining with each other (works in about 99% of content), or hold on to them if they have great Trust Masteries (getting each unit to 100% gives you some sort of ability/equipment that is usually way stronger than other gear). If the trust masteries stink or you don't need many of them, you can always fuse duplicate units into each other to raise their Trust Mastery % by 5%
There's probably some terminology you don't get yet in there, so feel free to ask if you're confused about anything.