It was my first time in Nether so I do not know much of what's happening there. I got chased by a piglin and fell in the lava and died so i decided to load a back up save file from before i entered. But when i entered the Nether again using my old save file, it showed the nether world of the one earlier and i could see the places i dug up and some of my items that didn't get in the lava were still there. I'm very confused here. Is this normal?
It depends on how you did the backup. Maybe you backupped only the overworld files, or by mistake you backed up another minecraft world entirely.
I don't reccommend doing backups this way (unless you do it for fear of loosing the world to device corruption). If you want to play with a safety net, begin with cheats activated (or open temporarily the game to LAN to activate commands) and use the /gamerule keepInventory true.
With this active, you're not going to loose your gear when you die. It may be considered “cheating” but to me it's ok while you're learning how to play (and the 1.16.x nether is deadly for beginners)
I have no experience in loading backups in this manner, but assuming you are using the Java edition my guess is that it is not normal. Your old backup should have no knowledge of your having entered the Nether if you really made the backup before going there. Maybe you thought you were loading the backup and loaded the current game instead? I can't think of any other cause, and am curious to hear what other people answer.
It's on how I use my back ups. When I load my back ups i simply open the world folder in the zip file and transfer all the files to the world folder in the saves folder, since the nether hasn't generated yet from my old save file, there will be nothing to overwrite the nether world in my saves folder so it will remain there unchanged.
Guess there's nothing left to this topic lol. Sorry.
I'll just delete the current save file that has doubled items in it.