indeed, if the name already exists in the user's folder it checks the contents of the file:
- exact same content? then it returns the same link as the old one, preventing having duplicates
- another file with same name? a number is appended to the name of the new file, to avoid mixing the new and the old

Files are never overwritten.
They are deleted if not used in a comment anywhere on the site ; so what is dangerous is keep for later in a text file a link to a pic that you uploaded but did not use already.
In such case the pic can be deleted (i run the cleaning function once every few months), and another pic could have re-used the name afterwards