I moved my website from one hosting to another one.
Then I edited the content (no the title) of one of my k2 item and after clicking the save button I got that message:
"Duplicate title alias entry detected. A number has been appended to the existing title alias to avoid URL conflicts. Alternatively, you can simply edit the title alias to your liking, as long as it doesn't conflict with another item's title alias."

It created a new item adding "-2" at the end of the url. I tried to remove the final string "-2" from Title alias box, but I always got the upper error message.
So now I have the same (edited) item but with a different url (I do not want). I cannot have a different url everytime I edit a k2 item.

If I browse the k2 items I cannot find the original item (the one without "-2"). It seems hidden. So I checked the DB and find it there. The old item is inside the DB with title field "Copy of bla bla bla" where bla bla bla is the title of my item.

k2 2.7.1
Joomla version 3.6.5
php version 5.6.30

Can someone help me?

So I checked the DB and find it there. The old item is inside the DB with title field "Copy of bla bla bla" where bla bla bla is the title of my item.

Is that item trashed? Can you try deleting that item?

You are right: it is trashed. I think I can delete it, but before deleting I would understand why when I save k2 items I get duplicates. Do you have an idea?

Duplicate aliases means issues :)
This is why you are getting this notice.
If you decide to drop the IDs then having duplicate aliases will mean that the first item with that given alias will be displayed, regardless if it is the correct item or not.