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it just a test mod

The core of the problem with the testlog is that you are both using SpawnPrefab to spawn a log which causes said log to run it's spawn code like normal, but then also return it in testlog's main function which will cause it to run the spawn code a second time.
This is likely to cause issues with almost any mod that uses PrefabPostInits as most are designed to only be ran once per object.