MySql command callback raised an exception (ArgumentException: An item with the same key has already been added. Key: 76561199015365735) at System.Collections.Generic.Dictionary`2[TKey,TValue].TryInsert (TKey key, TValue value, System.Collections.Generic.InsertionBehavior behavior) [0x000c1] in <437ba245d8404784b9fbab9b439ac908>:0 at System.Collections.Generic.Dictionary`2[TKey,TValue].Add (TKey key, TValue value) [0x00000] in <437ba245d8404784b9fbab9b439ac908>:0 at Oxide.Plugins.PlayerDatabase.m__4 (System.Collections.Generic.List`1[T] list) [0x00048] in :0 at Oxide.Core.MySql.Libraries.MySql+MySqlQuery+<>c__DisplayClass24_0.b__0 () [0x0005d] in <79d157ef50864cd6a493ea45b8d9c402>:0

What might the error be related to?

MySql command callback raised an exception (ArgumentException: An item with the same key has already been added. Key: 76561199015365735)
  at System.Collections.Generic.Dictionary`2[TKey,TValue].TryInsert (TKey key, TValue value, System.Collections.Generic.InsertionBehavior behavior) [0x000c1] in <437ba245d8404784b9fbab9b439ac908>:0
  at System.Collections.Generic.Dictionary`2[TKey,TValue].Add (TKey key, TValue value) [0x00000] in <437ba245d8404784b9fbab9b439ac908>:0
  at Oxide.Plugins.PlayerDatabase.<LoadMySQL>m__4 (System.Collections.Generic.List`1[T] list) [0x00048] in <d6c03fd24c1b45e082319b9a128877ae>:0
  at Oxide.Core.MySql.Libraries.MySql+MySqlQuery+<>c__DisplayClass24_0.<Handle>b__0 () [0x0005d] in <79d157ef50864cd6a493ea45b8d9c402>:0

It is trying to create a row in the database with the same key again 76561199015365735

You can try removing the row with that ID in your database and see if the issue disappears.