Many data compression methods fail to remove all redundancy from a clear-text file partly because they allow it to be encoded into many different compressed files. The compressed files are different in the sense that they are different sequences of bits. However, they are equivalent in the sense that, by decompressing any of them, we recover the clear-text file exactly. The existence of multiple encodings tends to increase the size of the compressed files. One of the causes behind multiplicity is the existence of equivalent messages.