The DSACK RFC is quite clear that you only send a DSACK block
once for every received packet. The current implementation of DSACK
on FreeBSD has a flaw in that it only uses new data from a client to
drive it to clean up a DSACK block. This is incorrect, and instead
after a successful transmit any DSACK blocks should be purged.
wIthout this fix once you receive (as a server) a DSACK block
you may get it multiple times.