a = c("sample1", "asd")
 b = c("sample2" ,"poua")
 c = c("sample3", "asd")

 dat <- rbind(a,b,c)

asd    sample1     sample3
 poua   sample2

tapply(dat[,1], dat[,2], as.vector)

$asd
[1] "sample1" "sample3"

$poua
[1] "sample2"

tapply(dat[,1], dat[,2], identity)

$asd
        a         c
"sample1" "sample3"

$poua
        b
"sample2"