So you probably have 2+ copies of the same system. You can see in the error it shows you the function's name "ItemTable_DropItems". Find that function in your code and delete it, or you probably want to delete the whole script and see what happens.
Keep in mind when importing Jass stuff that two authors can create functions with the same exact names. Like maybe author A created an Item Stacking system and named a function "ItemSetup" while author B created an Item Recipe system with the same exact function name "ItemSetup". If these aren't setup the correct way then you'll have issues. (there's methods to make code private so this issue doesn't happen)
Another big thing is that a lot of Jass systems require and use more popular systems such as TimerUtils. You need to be aware of what you're importing into your map, because you only want ONE TimerUtils. You could import 5 different spells from 5 different authors which ALL use TimerUtils and provide it. Now you have 5 copies of TimerUtils when you only need 1.