How to summarise unique values with respect to another coulmn?

You get more than one population value per continent because your data contains daily estimates of total population, not increments (I think), so you are keeping all value variations for each continent.

You first need to decide what criteria you are going to use to summarize the data, maybe the latest population estimate, or the mean over a period of time, or the range of population over a period of time, etc, etc.

If you need more specific help, please provide a proper REPRoducible EXample (reprex) illustrating your issue.