Errors shown in the minetweaker log show up in chat when you join a world. Since there are a lot of them, it is quite annoying.

Script used (Please Pastebin or gist your script, posting an unpasted or ungist'd script will automatically close this issue):

https://pastebin.com/QheNKEnh

Minetweaker.log file (Please Pastebin or gist your file, posting an unpasted or ungist'd file will automatically close this issue):

https://pastebin.com/MtaHw2sJ

Affected Versions (Do not use "latest"):

• Minecraft: 1.12.2
• Forge: 14.23.1.2582
• Crafttweaker: 4.0.12

Are you sure you linked the correct script?
The Erroring file is called MineTweakerRecipeMaker.zs

The Error you are getting is that you try to declare a variable that already has beed declared
e.g.

var myVariable = <ore:ingotIron>;
var myVariable = <ore:ingotIron>;

I renamed the file to script.zs and removed all the variables. This is the only file in my scripts folder.
New files:
script.zs: https://pastebin.com/kUUEBfuT
crafttweaker.log: https://pastebin.com/dDTBhxvW

After further research, I noticed that all the errors are from the same mod: Nuclearcraft. I went to report it to their issues page but found that someone else had already done so. It seems to be an issue between the latest version of CraftTweaker, and the recent versions of their mod.
tomdodd4598/NuclearCraft#206

NC issue