Uniqueness of checksums using DES-MAC encryption?
Think of it this way:


How many possible files are there?
How many possible checksums are there?


If the number of possible files is larger than the number of possible checksums, then there must be at least two files with the same checksum.

This is one of the things known as the "pigeon-hole principle"; the idea is that if you are placing pigeons in pigeon holes, and there are more pigeons than holes, then some hole somewhere will have two pigeons.

Now, if you go on further, you'll find that, if you know the MAC key, it's actually quite easy, with DES-MAC, to produce two files with the same checksum (or, for that matter, a file with a specific checksum).  However, you're not quite ready for that.