How do I retain duplicates and discount unique entries?

I have a list of customers who signed up for a free month. Some went on to pay for the second month and so appear twice because both the first free month and the second paid month are displayed. I'm trying to answer a question from Marketing: How many of those who signed up for the offer went on to pay for a month?

For example:

Customer 1 Paid £0
Customer 2 Paid £0
Customer 2 Paid £10
Customer 3 Paid £0
Customer 4 Paid £0
Customer 4 Paid £10
Customer 5 Paid £0

While what I want to get to is:

Customer 2 Paid £0
Customer 2 Paid £10
Customer 4 Paid £0
Customer 4 Paid £10

While I'm familliar with VLOOKUP and pivot tables, I haven't used macros. Is there anyone who can help?

Suggest using a countif function to identify the customers which appear twice and then filter.

=COUNTIF(A:A,A1)

in Cell C1 and copy down - assuming data is in A & B with no header