Copy and Paste issue inside timeline

I have a client who wants to watch while I edit her content so she can ask me to try numerous things. 

I have been copying and pasting to make it faster if she wants a certain word/font she likes the positioning of over a picture or video etc but sometimes the oddest thing keeps happening. If I copy and past the section - sometimes things that are deleted show up for a split second - like a phantom memory on the video? Am I doing something wrong here? There is nothing to delete - it makes no sense to me why it shows up on the copy and pasted section. 

In addition, there are sometimes / not always a few frames of black added to the beginning if I am looping the same footage. 

Any guidance appreciated.