Been a while, can't remember. Is it legal to take two of the same non unique upgrades on the same ship? Example: two glitterstims on a shadowcaster...

There are two possible restrictions on taking multiples of the same upgrade (provided the ship in question has the slots available).

The first is uniqueness, as you've pointed out. The second is "limited" which means "one per ship." For example: Tactician is Limited. A YV-666 could only take one copy. Alternately, Recon Specialist isn't limited, so a ship could take two or three of those and build a token stack. Glitterstim, to your point, is not limited. A Shadowcaster can take two.

There is one oddball upgrade worth mentioning here: Burnout SLAM. Currently, due to the wording on Burnout SLAM, if you SLAM, you'll discard both copies of it. That doesn't really have anything to do with your specific question, I'm just filling it out for completeness.

The other minor consideration involves cards that allow you to equip extra upgrades, but explicitly call out that they have to be different (like Royal Guard TIE, A-Wing Test Pilot, or Renegade Refit). Otherwise, as @theBitterFig stated, there's no restriction from taking more than one of a given upgrade (such as 2 Cluster Mines + Extra Munitions on Sol Sixxa).