ttk.Treeview.insert(... iid=None, ...) method has a check:
        if iid:
            res = self.tk.call(self._w, "insert", parent, index,
                "-id", iid, *opts)
        else:
            res = self.tk.call(self._w, "insert", parent, index, *opts)

Documentation says that "If iid is specified, it is used as the item identifier", but as you can see from code, iid is used only if it's "True". It means that you cannot use iids like 0, 0.0 etc.

This was initially reported in pydev thread "ttk.Treeview.insert() does not allow to insert item with iid=0" on 2018/3/16.  Igor, you should have mentioned on the thread that you had opened a tracker issue in response.  It was only happenstance that I did not open a duplicate.

I suggested there that 'if iid:' be replaced by 'if iid is not None:' but wondered if passing '"-id" ""' in the tk.call would be a problem.  If so, that would be a reason for the expanded check and suggest 'if iid in (None, ''):" as the replacement.

MRAB responded that '' is the id of the root of the tree.  But that does not tell me if passing a duplicate iid or that particular duplicate is a problem or not.

I have tested -- omitting the option and passing an empty string are treated different in Tk. Passing a duplicate iid is error. Thus the fix should be just `iid is not None`. But needed tests.

I made the changes on my machine and I\in my testing, it breaks on an empty string with an error saying
"Item already exists". If this is expected I would like to submit a patch with tests for the same.

Yes, this is expected.

New changeset 3ab44c0783eebdff687014f7d14d5dec59b6bd39 by Serhiy Storchaka (Garvit Khatri) in branch 'master':
bpo-33096: Fix ttk.Treeview.insert. (GH-6228)
https://github.com/python/cpython/commit/3ab44c0783eebdff687014f7d14d5dec59b6bd39

New changeset a7b1b847f665aafc22557917cea32ec34c9b4418 by Miss Islington (bot) in branch '3.7':
bpo-33096: Fix ttk.Treeview.insert. (GH-6228)
https://github.com/python/cpython/commit/a7b1b847f665aafc22557917cea32ec34c9b4418

New changeset 45116d393f713bbe918f16b33a0bba28b15bc96b by Miss Islington (bot) in branch '3.6':
bpo-33096: Fix ttk.Treeview.insert. (GH-6228)
https://github.com/python/cpython/commit/45116d393f713bbe918f16b33a0bba28b15bc96b

New changeset e80a232f2cfdab584133d9779c83885c5f9f1ba6 by Serhiy Storchaka in branch '2.7':
[2.7] bpo-33096: Fix ttk.Treeview.insert. (GH-6228) (GH-6326)
https://github.com/python/cpython/commit/e80a232f2cfdab584133d9779c83885c5f9f1ba6

New changeset 903f189b6e528cbe9500014c6f990c6511b38918 by Serhiy Storchaka in branch '2.7':
bpo-33096: Removed unintentionally backported from Python 3 Tkinter files. (GH-6724)
https://github.com/python/cpython/commit/903f189b6e528cbe9500014c6f990c6511b38918