Two creeps at the same position
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https://screeps.com/a/#!/history/E27S4?t=9948775
Two creeps can be seen on the same tile above the spawn at this time. This seemingly happened due to one spawning at a position while the other moved onto that position.
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I've seen the results of something similar twice today, both in the same place: E37N38, x: 29, y: 33.
I suspect it's the same cause as CursedFlames mentioned: creep moving into position at the same time as a creep spawns into it. It really snarls up my traffic management until one of them dies.