If you run a Google search for "KB915597 again" (without the quotes, of course), you'll get a boatload of hits.

Some of them are from people who are really having trouble with the Windows Update system. (It really is trying to install something over and over again and failing.)

But a LOT of them are from people who just haven't noticed this:

Microsoft is using the same KB number (KB915597) over and over again to deliver updates to the definition files of Windows Defender.

If you look closely, after the KB number, you'll see a Definition Update number that looks like 1.21.2529.3 or 1.22.3405.0 or some set of numbers in that format.

If yesterday you got a notice that Update KB915597 was ready for download and installation, you downloaded it and installed it, and you got a panel popup that said Installation Complete, ... but TODAY, there's KB915597 waiting to be downloaded and installed AGAIN ...

If you watch closely, you'll see that the definition numbers keep going up, and today's number is different from (higher than) yesterday's, and ...

There's nothing to worry about. The system is working, Windows Defender is updating as it should, and it's NOT the same fix coming at you over and over and over.