I have done a test of this with very similar settings to what you have used, I only had one resized image though. As a test I uploaded an image and checked the directory to ensure that is was uploaded and the resized version was named accordingly. I then went and repeated the action and checked the directory. I could see that the new file was uploaded with the increment value after the name. The resized version had the same name as the previous one but with the increment afterwards, here are the file names I was left with:
Is this not the same result that you are getting? Please post back with the details so we can help you get past this issue.