I have a little experience with GECK, before I was doing some little things in Construction Set for Morrowind, so I know some basics, but not cpecifics. So here's my question:
I was trying to revert changes made by one plugin - especially, it was one of head redesignes, and the object was Doc Mitchell. From what I know how to make this, I was needed to click on "details" button, find his ID, and tap "delete", thus setting him to "ignored" status, and then load and save the plugin. But after all of this operations, while loading plugin again in GECK, I saw that there now another Mitchell with "DUPLICATE" word in his ID. What is that? May I delete him? Or it will have an impact to my game? If so, what I have done wrong?

If you delete an NPC, that is bad.

I think what you are saying is you want to 'remove' changes to Doc Mitchell in a mod.

On the mod loader, click on the mod and make it the 'Active File'. Then click 'Details' button and find the NPC object for Doc Mitchell. Press delete here and it will ask if you want to really ignore these changes. Select yes. Load the plugin and save it and the changes to the Doc will be gone from that mod.

Sorry, seems my English is a hell of a gobbledegook. I actually made all the things you've just told me, but when I load changed plugin in GECK I get two Mitchells - one vanilla, and another changed, but with "duplicate" in ID.

If you have a duplicate DocMitchell, then you could probably delete it from within your mod. Since the game does not have a duplicate you can safely delete it.

I've not had much luck with the ignore feature in the NV GECK. If you want to remove accidental edits, I'd recommend NVEdit- not least because it'll highlight everything that's an edit for you, so it's easier to spot them.

I have not had any trouble wtih the 'ignore' in FONV GECK. I did the 'update facegen' thing to fix the sideways hat, and it 'edited' every single hat in the game. The ignore worked great to remove all those bogus edits.