If you use a saved mapping for an import, and an extra column gets added to the staging table after you load a .csv file, it may be a result of an action you took while originally saving the mapping. If, during your original import when you saved the mapping, you added a column to the staging table in step 2 of the import, this action gets saved with the mapping. Thus, every time you call upon that saved mapping a column gets added. The solution is to save a new mapping without adding a column during the process.