There was an article on the Cambridge Uni site a few years ago (sadly deleted now) which discussed the shift of the negative. It’s apparently such a common feature of language that you can “class” a language by where it is on the scale.
In Welsh, the hegative was originally "ni … ", so gwnes i - I did > Ni wnes i - I did not. A similar thing used to happen in French. Je suis - I am > Je ne suis - I am not.
For emphasis, you could add a dim or a pas.
But over time, the dim or pas becomes a part of the pattern, and you get the French Je ne suis pas, or the Welsh Ni wnes i ddim.
Over time, the natural shift is towards the ommission of the weaker ni / ne, in favour of the stronger, more emphasised ddim or pas. You see this all the time in spoken French, even though the French Academie keeps the full ne … pas in the official written form. Welsh has also been trhough this, and generally come out of the other side. Wnes i ddim is what you will generally hear and write.
However, where the na / ni / nid is still part of the sentence structure, you will sometimes get the middle ni … ddim, and sometimes the more original na without the ddim.
If you think “Oh no, that’s confusing - that will; make things difficult”, think again. Basically anything goes! Although you will *sometimes sound a bit formal leaving the ddim out, or a bit sloppy leaving the ni / na / nid out, it really doesn;t matter, because people are used to the three forms in different situations, and will understand you fully.
Personally, and I think this is a southern generality, I don’t usually use ni and ddim in the same negative , but I know plenty of people who do, so use what comes naturally, and adjust it to what you har most as you go about your daily life.