Filtering grouped data

Hi,

I'm trying to figure out repeat customers (where repeat customers are defined as customers making multiple transactions on different dates (i.e. so if a customer makes 3 transactions in one day but no other transactions they count as a new customer). 

Example data: 

So far I've duplicated the columns, removed everything except date & customer ID and grouped the data on customer ID so the data looks like this:

Using % repeats = calculate(count([customer ID]), unique transcation dates >1)/distinctcount([customer ID]) 

gives a perecent of repeat customers of 66% which is good

However, if I want to apply a filter in a report to look at transactions made in 2018, my measure doesn't change as the grouping removes the date column. In this case I want my grouped data to look like this:

Giving a % repeats of 50%.

Similarly if Customer 1 had made one transaction in 2018 and one in 2017 I'd still like them to appear as a repeat customer, as they've made a transaction in the year filtered for & have made bookings previously. 

Hope this makes sense, thank you in advance!

Sarah