The headings in a formatted table must be unique.

When you create a formatted table, it may change your headings row entries if you have a duplicated heading.

Maybe you have two Date columns. The first header will remain Date but the next column heading will be renamed to Date2 – see image below for an example. It will keep this numbering going if you have more duplicated headings.

This is a database requirement. In databases the columns (fields) must have unique names, otherwise things can get confusing.

This also applies to Power Query.

If you import an Excel table into Power Query it first converts the table into a formatted table and that will adjust those names to make sure they are unique.

If you import into Power Query from some other sources and promote the headers that will also ensure there are no duplicated column headings.