Minimum date in Cognos Report Studio

I have a list containing

[Customer ID]
[Transaction ID]
[Transaction Date]
[Sales Team Name]
[Purchase Type]

I want to have a list that gives me the date of earliest transaction for each customer. Filtering the report on:
[Transaction Date] = minimum ([Transaction Date] for [Customer ID])
will give me the earliest date the customer had a transaction.
The only issue with this is where the customer has two transactions on their first day. That’s the bit i’m working on. Any help or advice would be greatly appreciated.