I have had a couple of people come to me to fix self tanning streaks that they got when using high street brands. I went over the areas which were dark with moisturiser and then resprayed them. I only did this because they were desperate and did tell them I couldn't guarantee it would work and would probably not fade well where the original tan was. They seemed happy with the results although I didn't see it myself, they were to be mother of the bride and bridesmaid the next day, so really did need help.
I have also had the occasional client who has streaked their hips when pulling on tight trousers (these were like lots of really thin vertical lines, like they had been scratching) I just misted over this area and it was fine the next day.
You will never be able to guarantee that the results will be perfect, but you will do the best that you can.
Hope that helps xx