Disturbing disk catalog oddities

Two things have happened recently with my late 2013 MacBook Pro on Mojave 10.14.6 that are giving me concern.

(1) I opened a text file with all my notes about my crypto wallet and found not these notes, but rather some find results about an award a college friend had won. This was of course disturbing, as I didn't remember what had been in the file and it might have been very important, like for instance necessary for access to my crypto. I deleted the odd text, added a new sentence or two noting what had happened, and did a save. Sometime later, I noticed a file named "herlihy myar t.o.txt" in the same folder that would have held the find results about the award my friend (Herlihy) had won. The bizarre thing was that this file now contained the same new text as the crypto wallet notes file, with the same modified date. I certainly had not put the same text into two files, not even having been aware of the second one.

(2) Not long ago today I downloaded a PDF I named "Flip City #13.pdf" to a folder already containing "Flip City #12.pdf". When I checked the contents of the folder, it contained two copies of the latter with precisely the same name (I compared them in a text editor to be sure). I wondered how was this possible and whether this and the other thing indicated that my disk was screwed up somehow. When I looked at the folder again just now the duplicate had disappeared, but its having been there even temporarily was still disconcerting, particularly in light of the earlier problem.

I'm reporting this in the hope someone will understand what's happened here, be able to explain it to me and advise me on anything I should perhaps do (also maybe to recover my crypto wallet notes, though I doubt that's possible). I've already run Disk Utility First Aid on the volume and it said it didn't find any problems and the disk appeared to be okay. I suspect there's something not okay if it's doing things like this, however. Thanks for any help.