stuck with repeated values

R Gurus,I have a vector of nearly 90,000 characters from which I have to
extract the index of the characters which are repeated. So suppose if
x<-c("a","a","b","a","b","c","d") then my output would be a vector having
the index where the values are repeated i.e (1,2,3,4,5). I have been able to
isolate out the values that are repeated from the unique list of characters
in x (i.e. y<-c("a","b")). One method can be to use a loop on the which
function but that would be too time consuming. Any hint on using the apply
function effectively would be useful
Thanks and Regards

Souvik Bandyopadhyay
Lecturer,
Dept of Biostatistics,
Indian Institute of Public Health,
Hyderabad

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