If item value is repeated for another item, make only one call?

If item value is repeated for another item, make only one call?

Let's say I have a list of usernames like this:
<li>burgerlover29 - 5:00</li>
<li>purplepeople8 - 8:04</li>
<li>laughingcathy - 9:10</li>
<li>purplepeople8 - 11:07</li>
for each item, I make a call with the username to get their first name.
What is the best way to avoid making multiple calls for a user whose
information I've already gotten?
ex. I call for purplepeople8 once and then his name is updated for that
item and the 4th.
Have had to get the name after the list is written out because the list
provided by another call.