they say, all files will be client-side encrypted which won't allow them to "deduplicate" anything.
Mega might be doing the same thing: saying one thing to attract early adopters, and changing the marketing language once it gets broader adoption by people who don't care about that attribute. It's dishonest, but it would hardly be shocking to learn that a multiple felon was being dishonest.
This would allow them to do deduplication at the block level (see ZFS for example).
What's the probability of two 4KB (or whatever) blocks of random data being identical? Basically zero even with petabytes of data.
The encryption on the client doesn't use a random key. The key is a hash of the unencrypted contents of the file.
If you upload the map to the rayiner family treasure that only you have seen you're good. No one else will be able to read it.
But if you upload the latest episode of Modern Family and Disney gets ahold of the same rip you used they (if they can get a government to help them out) can see what you did and charge you with copyright infringement (or whatever the appropriate crime would be).