I've had this sv error happen a few times over the last few days from different players, not sure how to exactly trigger it but i assume it's somehow getting through the dupe sanitization.
addons/advdupe2/lua/weapons/gmod_tool/stools/advdupe2.lua:192: attempt to index field 'HeadEnt' (a nil value)
1. __index - [C]:-1
2. GetDupeElevation - addons/advdupe2/lua/weapons/gmod_tool/stools/advdupe2.lua:192
3. LeftClick - addons/advdupe2/lua/weapons/gmod_tool/stools/advdupe2.lua:215
4. <unknown> - gamemodes/sandbox/entities/weapons/gmod_tool/shared.lua:226