Some of these responses puzzle me. I have no trouble displaying just the measurements I want to show, turning others off, adding new ones or displaying old, previously turned off measurements. Of course I actually use the scope every day to make measurements (however quirky or wrong or misspelled they may turn out to be) so I am very familiar with how the system operates. It appears that some people weren't even aware of the "Select Item" list or haven't used it very much. I suggest playing around with this feature a bit; eventually it will "sink in" and you'll be able to use it to display what you want, without being troubled by the "greyed out, inactive but displayed anyway" problem. Use the "Select Item" list to remove undesired greyed-out items from the screen display.
Perhaps it may help to think of the measurement display as a sort of list buffer that holds the last five that you have selected and allows you to turn on or off any of those using the "Select Item" list. When you want to add a sixth one, it goes in the fifth place and pushes the rest down one position, and the old "first" one is gone. It gets slightly more complicated when you have Large or ExtraLarge fonts selected since only three or two of the five can be displayed at one time, or if you insist on displaying your chosen measurements in some particular order.
This means when you want to add a measurement that is already displayed as invisible measure "4" or "5", the scope says "measurement already added". The only way to display that measure, is first to add 2 more other measurements that you do not need, and then select the one you need again...
No, that's not right either, if I am understanding you correctly. You just need to use the "Select Item" list to turn off one of the measurements of the three that are displayed, then you can turn on the one you need again.
Sure, the Rigol software often appears to be a set of "kludges" that were cobbled together by a committee of junior programmers who have never actually used an oscilloscope... but who remember playing with those complicated Chinese puzzle boxes as kids.
But what did you expect, it's only 400 dollars.
Shouldn't you be happy that it even "pretends" to do measurements at all?