SandForce presumably uses some sort of differential information
update. When a block is modified, you find the difference between the
old data and the new data. If the difference is small, you can just
encode it over a smaller number of bits in the flash page. If you do the
difference encoding, you cannot gc the old data unless you reassemble
and rewrite the new data to a different location.

Difference encoding requires more time (extra read, processing, etc).
So, you must not do it when the write buffer is close to full. You can
always choose whether or not you do differential encoding.

It is definitely not deduplication. You can think of it as compression.