When I delete on the left side it deletes on the right side... (help) (Zmodeler3)

16BitShooter

Alright, to start off I'm extremely new to this so if this is something very nooby please forgive me.
For the past hour or so I've been fitting this bumper I found to a Dodge Dakota. I noticed the inner fender would look very weird if I didn't take some if it off.

With no edits;
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You see the inner fender still has some junk on the front that isn't needed with this bumper. So I removed it for it to look better overall;
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My problem is that I'm trying to duplicate it on the left side, but when I attempt to remove a polygon on the left it removes it on the right side. It's weird too, it takes a piece that I don't want to remove... Any and all help would be great! Thanks!

Edit: Also, If anyone has a rear bumper they don't mind letting me use for this that would be great!