Currently, the rule lives under
unique and universal, but it was already around when I started studying the rules really hard (so it's from before 2013):
When any card marked “Not duplicatable” in
its game text (or any unique time location)
is in play, another copy may not be seeded
or played by any player, and, if another copy
is encountered, earned, or activated by any
player, that copy is discarded. When a player
seeds any unique mission which has already
been seeded on the spaceline, the second
copy is stacked atop the first to create a
single location. See Deep Space 9, seed
phase.
Originally, Time Locations were treated basically the same way as Missions -- which makes sense, since they're both forming distinct, shared locations. Back in the old days, if your opponent seeded a Mission, and you tried to seed a copy of the same Mission, your copy was discarded, because Missions can't be duplicated. (You then had to download a universal Mission from outside the game to replace it.) Same with Time Locations: your opponent seeds it, then that location exists now, you can't duplicate it, and you have to throw your copy away. (Most time locations are carefully worded to account for this.)
But Missions were changed in the year 2000 to the modern rule of "shared missions": now, if you both seed the same mission, you stick them on top of each other and treat the location as seeded by both players. Time Locations never received the same update, though. They're still treated today the way missions were in 1999: if your opponent seeds it, yours is gone. I can't personally say I understand why that is, but that's still the rule all the same.
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