Creating a subset based on ID numbers

Hi everyone,

I'm new to R and statistic in general. I have got a dataset, where I'm going to make a Kaplan-Meier plot. Before that, I have some trouble regarding basic data manipulation.
The problem is that every row contains a new measurement on different dates. Every row has a User.ID number, but each user may have reported more than once and therefore the same number may appear multiple times (see figure).

Is it possible to sample User.ID, so R will recognize every number as an individual with more than one measurement? If this is the case, how can I do it?
Thank you in advance! :smiley:

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