Sum with condition

I guess my problem is simple for most of you but I am new with R and I need
some help,
I have a dataframe : 

CELLCD AreaProtected
   8928      52.39389
   8928      41.91511
   8929      21.21975
   8929      63.65925
   8930      26.08547
   8930      14.04602

I wouldlike to sum the AreaProtected if it is the same CELLCD in another
column :

CELLCD AreaProtected    SumAreaProtected
   8928      52.39389      94.309
   8928      41.91511
   8929      21.21975      84,879
   8929      63.65925
   8930      26.08547
   8930      14.04602

I am just started with R and I don't know how I can do that.
Do you have any ideas ?


Thanks a lot for your help,

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