Cant you get something at a point in time, and then grab it at an earlier point in time to have two? but not the other way around?
With the sealed chests, the trick is to first inspect them in 600 AD. You'll get a message saying that the contents are reacting to the pendant, then you're given the option to open the chest. Refuse, then go to 1000 AD and open it. You'll get a powered-up version of the item (since it's been percolating in Lavos energy for 400 years). You can then go back to 600 AD and open the chest to get the original version as well.
If for some reason you don't want the powered-up item, you can instead get two copies of the original item by simply not inspecting the chest in 600 AD first. Just collect the item in 1000 AD, then go back to 600 AD for another copy.
If you open a chest in 600 AD without first opening it in 1000 AD, you'll only get the one item, since the chest will disappear afterward.
There are a few sealed chests that only exist in one time period, so you can't power them up or open them multiple times. These chests are in the Heckran Cave, the Forest Ruins, and the Magic Cave, unless I'm forgetting some.
