Generally: 
I recommend to use the latest version of Plex and the Plex Behaviour 
that ships with Remote Buddy. This works fine out of the box.

With "Single key press between press & release", Remote Buddy emulates 
a key press when you press the button on the remote and a key release 
when you release the button of the remote. While it doesn't send 
repeat events explicitely like "Single key press between press & 
release with repetitions", many media applications install a timer on 
receipt of a key press and "emulate" their own "repeats" until the key 
is released. The idea behind that from an application's developer 
perspective is independance from the key repetition rate that can be 
different on a per user basis.

In order to prevent repeats - also target-application-internal ones 
(which is likely what you are seeing) - completely, you should select 
"Single key press on press". That will make Remote Buddy send a single 
key press and release immediately after one another when you press the 
button.

Best regards, 
Felix Schwarz