How to sum the count of three users?

Hi, in my scenario i have a lot of users for example: user1, user2, user3... and i want to count their logins to a server and create a pie chart. I have the problem that there are some users which have the same name but a different notation for example user1, USER1 and USer1. How can i sum the counts of these 3, allocate it to user1 and create the pie chart? Thx for your help!