Need help figuring out how to avoid sending the same email twice in a flow that has a conditional split

Hi,

We have set up a post-purchase flow for our 1 time buyers (its a 3 months flow with approx 10-12 emails). However we are trying to add a conditional split where - if the customer buys a 2nd time - they will go into a different flow. In this 2nd time buyers flow, some of the emails are the same as 1st time buyers flow. How do we avoid sending duplicate emails to customers in the 2nd flow? 

Is there a way to ensure that customers dont receive duplicated emails? 

I hope my question makes sense

Thanks 

Tan