This seems in line with the rest of the games effects, you generally can't stack the same debuff even if it's from two players.
also AOE-DOT affects should not stack (from the same source, because that's way too strong AOE. If you had refresher, and echo shard you can effectivily stop people from being able to push an area for a long time, which would not make sense for this game.
The game is build on the idea of press your advantages or lose then, but if you can area deny for 15-20 seconds, then it straight up breaks the flow.