[R-sig-Geo] Number of duplicated pixels

[Komine]

HI,
I have a matrix containing 2 columns: coordinates X (longitude), Y
(Latitude), representing the coordinates of burned pixels for 10 years. I
want to known the pixels which burned once, twice, three times etc. in the
10 years. So I would like to obtain the frequency of the burned pixels. 
I used this following code which is good. 

> essai[which(duplicated(essai,incomparables=FALSE)==TRUE),]

But I want to know the length of each duplicated coordinate. I used this
code with length: 

> length(essai[which(duplicated(essai,incomparables=FALSE)==TRUE),])

But it shows only the length of the first duplicated coordinate. 

Can you help me to resolve it!
Thank you in advance 
Komine 

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