I have to disagree when it comes to beltloops.  When you're being reasonable about things, it is not bad to plan activities that can fulfill multiple requirements at the same time.

For the Webelos, there are some requirements that specifically say if you have done this for one advancement, it cannot count for this other one so you have to do it a second time.  That's the only place that double-dipping is specifically mentioned that I'm aware of from Tigers up through Webelos.  In Boy Scouts I am sure that doing something once and counting it towards multiple requirements is not allowed but that's an entirely different program with different aims and methods. 

If you're a Wolf den and you are working on section 1j of the book (teaching the three types of basketball passes – bounce, chest & overhand), would it be inappropriate to also teach them the rules of the game and then play a game of basketball (completing the basketball beltloop)?  Not at all!  And in doing that, you have also completed the requirement of Wolf elective 20m, to show how to shoot, pass & dribble a basketball and take part in a game.  Should the boys get credit for all three of these things during this activity or should you make them do it all a second time?

Our role as leader is to get these boys to experience these things (some things are new, some aren't).  In doing so it builds values and character and teaches them to try things they may have an interest in doing.  Especially at the Cub level, it is very much about encouraging them to explore the world around them, learn about all the opportunities available to them.  One of these methods is through recognition and advancement used to keep them motivated and moving forward.

The key here is to plan it out and that way you can make the boys aware at the time of that they are fulfilling multiple requirements.  Using the example I gave above, you can then tell your boys at the beginning of the meeting, "Today we're going to be learning basketball.  We're going to learn about the rules, practice the skills needed and even play a game!  When we're done you'll have completed part of your Wolf rank work, an elective and earned a beltloop!"  How cool is that meeting?  The boys will meet the intent of all of those three requirements and will walk away with something to show for it.  To a 7 or 8 year old, what could be better?  Getting to play ball with his buddies and getting a new shiny for his uniform!