First date where userID appears in dataframe in R
<p>I want to group data by rows in a dataframe. Each user has a unique userID, and each user might appear several times in the dataframe. There is a column for each time they appear. So, for example, user01 might have logged in on 10-01-2014 and 10-02-2014, so user01 will appear in two rows.</p>

<p>I want the first instance each user appeared. How can I do this in R or Excel? I feel like this would be easy to do in SQL, so I must be missing something obvious...</p>