change column value only if two other columns are duplicates - r

Question

I am having a hard time to figure this out in R.
This is what I would like to do.
In a data frame like below, I would like to do if Name and Class duplicates add two row's score and if not, leave it as it is.
+------------------+-----------+-------+
| Name | Class | Score |
+------------------+-----------+-------+
| Sara | Sophomore | 10 |
| John | Freshman | 20 |
| Taylor | Sophomore | 30 |
| Tyler | Junior | 10 |
| Keith | Junior | 20 |
| Andrew | Senior | 30 |
| Victor | Senior | 10 |
| Nancy |Sophomore | 20 |
| Taylor | Junior | 30 |
| John | Senior | 10 |
| Victor | Freshman | 20 |
| Sara | Sophomore | 30 |
| John | Freshman | 10 |
| Taylor | Sophomore | 20 |
| John | Senior | 30 |
+------------------+-----------+-------+
So basically, the end result should look like:
+--------+-----------+-------+--+--+--+--+
| Name | Class | Score | | | | |
+--------+-----------+-------+--+--+--+--+
| Sara | Sophomore | 40 | | | | |
| John | Freshman | 30 | | | | |
| Taylor | Sophomore | 50 | | | | |
| Tyler | Junior | 10 | | | | |
| Keith | Junior | 20 | | | | |
| Andrew | Senior | 30 | | | | |
| Victor | Senior | 10 | | | | |
| Nancy | Sophomore | 20 | | | | |
| Taylor | Junior | 30 | | | | |
| John | Senior | 40 | | | | |
| Victor | Freshman | 20 | | | | |
+--------+-----------+-------+--+--+--+--+
As you see if name is the only duplicated value, it does not change (Example of John Freshman and John Senior). If class is the only duplicated value, it does not change either... Two columns in a row have to be duplicated to make the change.
My try is as below, but it is not working and am getting error message
'Error in if ((experiment[i, 1] == experiment[j, 1]) & (experiment[i, 2] == : missing value where TRUE/FALSE needed'
My code:
# creating an empty data frame
experiment1<-data.frame(matrix(ncol=3, nrow=15))
for(i in 1: nrow(experiment)){
for(j in i+1: nrow(experiment)){
if((experiment[i,1] == experiment[j,1]) & (experiment[i,2] == experiment[j,2])){
experiment1[i,1] <- experiment[i,1]
experiment1[i,2] <- experiment[i,2]
experiment1[i,3] <- experiment[i,3] + experiment[j,3]}
else{
experiment1[i,1] <- experiment[i,1]
experiment1[i,2] <- experiment[i,2]
experiment1[i,3] <- experiment[i,3]}}}
Could anyone help fixing my code or figuring out "nobler" code?

Answer 1

Aggregation is like the first argument explained in any basic R tutorial, I suggest you go and follow some.
base R
aggregate(formula = Score ~ Name + Class, data = mydf, FUN = sum)
dplyr
mydf %>% group_by(Name, Class) %>% summarize(scoreSum = sum(Score))
data.table
setDT(mydf)[ , .(scoreSum = sum(number)), by = .(Name, Class)]