If you attach an object that shares a name with another object that has been attached, you will get that warning. It isn't really an error, just notification that the latest object "blocks" the name of a previous object. Personally, I never use attach().
DF <- data.frame(Col1=1:3)
DF2 <- data.frame(Col0=11:13,Col1=21:23)
attach(DF)
attach(DF2) #the name Col1 is in both data frames.
The following object is masked from DF:
Col1