Q:

Why am I sometimes having "doubled image" and soft photos with my Tamron 100-400mm

I sometimes get unsharp results out of my Tamron 100-400mm F/4.5-6.3 Di VC USD, with a "doubled image effect", especially visible in the out-of-focus area (bokeh).
But the rest of the time, I get really sharp results and I'm quite happy with those.
This first image was taken at 400mm f/6.3 1/320 ISO1600 with VC activated. The pony isn't quite sharp and we can clearly see a "doubled image" on the white electric fence behind the pony as well as "lines" in the overall bokeh.

This second image was taken at 400mm f/6.3 1/1250 ISO1000 with VC activated. It is totally sharp and there is no "doubled image" artifact in the bokeh.

Here is the 100% crop comparison:

Both images were taken with my Canon 70D.
I can't identify what is causing this, it can't be a focus issue since this issue is present on the whole image, in and out of focus. It is not motion blur because I have seen this issues in images with shutter speeds as fast as 1/800, which should be more than enough even without VC.
I have suspected VC to be the cause, I have read that it could happen when releasing the shutter too fast after enabling the VC, but I always half press the release button at least one second before shooting.
I have tried to recreate this on purpose by taking pictures as fast as possible after starting VC, by shaking the lens before the picture to see if maybe some element could get misaligned, but I couldn't recreate this.
I would be grateful for some insight because I have been really happy with the overall performance of this lens. Thank you.

A:

I seem have found the culprit, my single coated 67mm Hama UV filter.
I put it on as some point between these two pictures and didn't think it could be the cause because I have never had issues with filters before with any lens.
Here are before and after pictures with and without the filter, both at 400mm f/6.3 1/400 ISO250 on my 70D.

And 100% crop comparison:

EDIT: Since comments pointed it out, I did move a little bit to the left between those shots, was using autofocus, and was handholding my camera. This doesn't make for the most rigorous comparison but focus can't be blamed since we clearly see a difference in the bokeh which are of course both out of focus by definition. The "doubled image" effect isn't flagrant at the plane of focus, we can mostly see a more blurry image, but we clealy see some diagonal lines in the bokeh that are cause by this "doubled image".
Here is another comparison, this time mounted on a tripod and without changing the focus between shots, only removing the filter:

I am relieved that my lens is not faulty but am surprised of the quality deterioration that this filter is bringing.
I have often heard that cheap filters could degrade IQ, but I had never seen it before with any of my lenses. I will be returning this filter, hoping it was a faulty one, but I found funny to find this in the product description:

Impact on the pictures:
Clear, sharp photos
Reduces haze and prevents fuzziness

