Dynamic Column Name (M/PowerQuery) Difficulty

Hi all,

I am importing many files (with the same structure from a folder). These files automatically append to each other.

I required to change  one of the column names based on it's index position. (as when new files are added to the folder I can't be certain the same file is going to be taken as the first file. This could lead to the column header name to being different.

= Table.RenameColumns(#"Promoted Headers",{{Table.ColumnNames(#"Promoted Headers"){28}, "--1"}})

I succesfully changed the column header name. Everything is good up until this point! 

However, if the column name is already --1. (The name I am trying to change to) PowerBI will return an error saying:

Expression.Error: The field '--1' already exists in the record.
Details:
Name=--1
Value=

I considered using a conditional formula inside of this where I would say If = -1, take the previous query. This isn't working and even if it is, I need to replicate this rename column for 5 columns. I can't take the previous query because it depends on which ones worked.

Sorry If this isn't clear, I can try clarify if the question again.

Thank you