Duplicate unpublished posts appearing

I’m a new WP user (1 week) and this seems like a bug to me but I could just be doing something silly…

Problem:
While writing posts, WP keeps automatically saving my post and creating duplicates of the post as I write it at different stages.

Conditions:
I have edited/renamed the default category so it is not called “Uncategorized” any more. I am using Excerpts and custom fields (I intend to use these values later from custom templates). Other conditions exist of course, however I reckon one of these is the cause.

Steps to reproduce:
1: Click “Write”.
2: Type in “My post title” in the title field.
3: Type “My post goes here” in the post field.
4: Select a single category other than the default category.
5: Type in “My excerpt goes here” in the excerpt field.
6: Add two custom fields (type in key names first time then subsequently select from drop-down).
7: Click “Publish”

Result:
In addition to my correctly published post, there are two new unpublished posts, one titled “Draft created on <date> at <time>” in the default category and a second titled “My post title” in a non-existent category called “Uncategorized”.

Result variations:
1: Occasionally there is also a third unpublished post titled “My post title” in the correct selected category (as well as the published post).
2: By omitting step 6 (adding two custom fields) only a single extra unpublished post with title “(no title)” is created in the non-existent category “Uncategorized”

Suspected cause:
I haven’t delved into the WP code yet but I suspect that “Uncategorized” is hardcoded somewhere in the code, and that renaming the default category causes this bug to manifest. I also suspect that when you add a custom field it makes a draft copy of the post under these conditions.