I've recently been getting failures on my Defender updates. When Windows checks it is often showing two updates as available and it seems it then goes on to try and install both together.
I should point out that I have Windows to check only for updates and for Defender I have a 7.30am task set to install the definitions. This has worked well for months but suddenly something seems different with the updates. I often two Defender updates waiting. The Event Viewer log shows the timeline which happens all at once.