For many of FFG's games, one of the most common reccomendations for what to buy after the core set is another core set. In the LCGs, this gets you extra copies of all the cards, including some very powerful ones. Even before X-wing gotba second core set, imperial players often bought two since buying two tie fighters and a dice pack only cost two dollars less than a core set that came with all that as well as spare templates, an extra damage deck, and an X-wing you could trade for something. Armada, however, broke the model. Each faction only got ships worth less than half of the coresets value, and while another maneuver tool and more dice are one of the more common purchases, those two inexpensive purchases and a handful of redundant fighters weren't worth 60/70 dollars. So, how does Legion measure up?
At the most basic level, a duplicate legion core set gives you $75 of usable miniatures (Two trooper units and a Support unit, each worth $25). You also get an extra Commander, but until we get a Luuke Skywalker commander that's not much use aside from conversions and displays. Factoring in the almost essential extra set of damage dice ($15 right there), a spare set of maneuver tools and objective cards, and some more Barriccades for the terrain starved, and at the surface level you've got a pretty good deal. However, it's not quite that simple.
For one thing, do you really want all those units? While you will need a third corps unit, both faction have another type of corps unit on the way. Maybe you want some Snow Troopers who won't leave Darth Vader behind, or the powerful close range blasters of the fleet troopers. Or maybe you'd rather an AT-ST and Veers instead of more speeder bikes. Another thing to consider is that some cards are unique to the individual expansions- The corps unit expansions comes with copies of the Impact Grenades and Climbing Hooks upgrades, and it appears that the Speeder bikes and AT-RT also come with at least one unique upgrade. Even if you do want a ton of Stormtroopers, you'll probably want at least a few copies of impact grenades to repel angry AT-RTs trying to squish your troopers in a melee.
So where does that leave us? If you do want all the units from the core set, It's still probably a good deal, especially if you plan to get even more of those units or plan to play both factions. Still, forewarned is forearmed, and if you plan a bit ahead, you figure out what the best deal for you is.