1/ The specified item was not found. is not for product related questions, but for generic marketing ones. as Gerard suggested, please move your question to Products
2/ You will have to look at the activity log of these leads to understand what's happening. And in order to help you, a lot of info is missing, starting with what CRM are you using, and what do you mean by syncing twice Does that mean you end up with duplicates in your CRM ?