Does online backup replace files, or add them again?

I have Norton 360 with 2Gb of free online storage.  The backup set I created to be backed up contains about 1.5 Gb of data.  The first backup went well, but after the second backup, Norton told me I have no space left.  I know I did not create anywhere near .5 Gb of new files.  I mostly change existing files.

 

When I went to delete some files from online backup, I noticed that there were identical file names listed for the two different backup dates.  When a file is changed without changing its name, is it replaced in the online backup, or is it added? 

 

If updated files added as if they were new, I would have to constantly buy more space, or spend precious time deleting older versions of documents.  Either way, it is unacceptable.

 

Can anyone answer this?  Thank you.