R: count elements in column and combine without loop/rearrange

Question

to summarize my problem i got a dataframe like:

1   10
3   20
5   40
6   30 
1   100
2   25
7   13
3   70

Like for an ID in the 1st and a price in the 2nd column. I need an output like:

1   110
3   90
5   40
6   30 
1   110
2   25
7   13
3   90

So the sum of all prices with same ID without any loops, without rearranging or compressing. I don't want solutions like:

1  110
2  25
3  90
5  40
6  30
7  13

And this one is also not helpful:

 for(i in 1:8){
   x=0
   for(j in 1:8){
     if(df[i,1]==df[j,1]){x=x+df[j,2]}}
   df[i,3]=x
 }
df[,2]=df[,3]

cause it only works for small dataframes (df).

I guess the solution is simple... but anyhow I could not make it..

Thank you in advance :)