counting unique factors in r

You could also run unique on the data first:

aggregate(dam ~ bdate, data=unique(mydf[c("dam","date")]), FUN=length)

Then you could also use table instead of aggregate, though the output is a little different.

> table(unique(mydf[c("dam","date")])$bdate)

2009-10-01 2009-10-03 
         2          2

What about:

aggregate(dam ~ bdate, data=mydf, FUN=function(x) length(unique(x)))

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