I’m brand new to Legion, so maybe I missed this easy question. If I have a unit of Stormtrooper with Precise1, and I take the targeting scopes upgrade. Does that give them a further precise 1 for a total precise 2? In general when you have 2 effects granting the same keyword ability does it stack?

Generally yes, numbered special rules do stack. So stormtroopers with targeting scopes get to reroll 4 die instead of the normal 3(2 for aim and 1 for precise) base.