a_ply warning: duplicated names/ levels

[mhische-md]

When running the following code:

library( "plyr" ) # installed version is 1.8.4
A <- matrix( as.logical(c(1,0,0,0,1,1,0,1,0,0,0,1)), nrow = 3, dimnames = list( LETTERS[1:3], letters[1:4]))
A.idx <- which(A, arr.ind = TRUE)
a_ply( A.idx, 1, function(x){ print(str(x)) })

I recieve two warnings:

1: Duplicate names not supported.
2: In levels<-(*tmp*, value = if (nl == nL) as.character(labels) else paste0(labels, : duplicated levels in factors are deprecated

However, I can not see duplicated names in my data.
Could anyone please give me a hint, what these warnings are about or what I am expected to change here?