Someone asked about this today so I figured I’d make a forum post in case anybody comes searching for it later…
There’s a known issue with DuckClient and the keepalive feature. It can cause lines of text to just disappear from your screen. A few folks have said it can also cause messages to be delayed or duplicated, but I personally haven’t witnessed those behaviors. The “lines get eaten” one is easily reproducible though.
To fix this, just turn the client keepalive feature off: keepalive off. You’ll have to do this on all of your characters if you have alts.
Only DuckClient seems to have this problem and I’ve reported it to the developer.
This and other client configuration tidbits (including how to set up the Edit feature and enable special characters) can be found in the Client Setup tutorial.