Okay, so I have a late 2013 model retina macbook pro. A couple a days ago i placed a sensitive file on the desktop, and later deleted it by emptying the trashcan. I then read that was not the way to get rid of a sensitive file. I still have a copy on the file stored, on my other computer, and I was wondering if putting the same file in the same place (the desktop) would overwrite the old file I had deleted which now is allocated as "free space". I can use disk utility because I have an SSD. Then I can do a secure delete in order to erase it completely.
There is no way to guarantee to put the data in the 'same place', no matter where in the filesystem you put it.
Hard drives just don't work that way. They will put specific data anywhere they like [or to an algorithm unfathomable to mere users]
To be reasonably certain of overwriting the data [with as much certainty as you can have without destroying the drive] you would use Erase Free Space - which you will have noticed you cannot do on an SSD - though the way SSDs use their storage is completely unlike a HD; they try to allocate writes all across their memory-space to even out the wear. This makes recovery of a deleted file a much more difficult task than with an HD, but not impossible.
As far as I'm aware [& nothing ever seems certain in these things] the best way to 'erase free space' on an SSD is to fill the drive with random data - anything, a movie file copied over & over til the disk is full, then delete it all again.
That will have written over the previous data at least.
Repeat ad nauseum for multiple pass erase [if that even really has any meaning for an SSD.