[R] Summing identical IDs

[PDXRugger]

Hello All, 
   I would like to select records with identical IDs then sum an attribute
then and return them to the data frame as a single record.  Please consider


Acres<-c(100,101,100,130,156,.5,293,300,.09)
Bldgid<-c(1,2,3,4,5,5,6,7,7)

DF=cbind(Acres,Bldgid)
DF<-as.data.frame(DF)

So that:

  Acres Bldgid
1 100.00      1
2 101.00      2
3 100.00      3
4 130.00      4
5 156.00      5
6   0.50      5
7 293.00      6
8 300.00      7
9   0.09      7

Becomes

  Acres Bldgid
1 100.00      1
2 101.00      2
3 100.00      3
4 130.00      4
5 156.50      5
7 293.00      6
8 300.09      7

dup<-unique(DF$Bldgid[duplicated(Bldgid)])
dupbuild<-DF[DF$Bldgid %in% dup,]
dupbuild..dupareasum<-sum(dupbuild$Acres[duplicated(dupbuild$Bldgid)])

This sums the unique Ids of the duplicated records, not whati want.  Thanks
ahead of time

JR


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