The reason why there are two is because you receive this Burtgang at the end of the third quest before the fight with Balrahn, and all of it's attributes are not unlocked yet.
Burtgang RareExclusive
(Sword) All Races
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DMG: 53 Delay 264
Enmity + 12
Physical Damage Taken -12%
Lv. 80 PLD
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DMG: 60 Delay 264
Enmity + 14
Physical Damage Taken -14%
Lv. 85 PLD
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DMG: 67 Delay 264
Enmity + 16
Physical Damage Taken -16%
Lv. 90 PLD
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DMG: 74 Delay 264
Enmity + 18
Physical Damage Taken -18%
Lv. 95 PLD
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DMG: 81 Delay 264
Enmity + 20
Physical Damage Taken -20%
Lv. 99 PLD
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Wishful thinking I think not, if it continues to follow the path it has this will likely be the path it goes.
Since it upgrades every 5 level currently one would assume it would stop upgrades at 95, however this is a max level weapon therefore there will likely be a 99 version.
I'm curious to see how close my assumptions will be.