Do we need both these tags? Is this a good tag synonym? What's the scope of either one of these tags, or both?
The difference may be subtle. Let's choose character development as example:
You can plan your characters. You can develop your characters.
A character cannot plan, but a character can develop.
Planning is the phase you do up-front. Developing continues over the whole project. You can plan your character to be Marlon Brando, but then while writing he develops to be Clint Eastwood.
Planning is easy and undisturbed. Development can be chaotic if nothing goes as you've planned it. Development may need to be controlled (you really want Marlon) or be enjoyed as creative process (Clint fits so much better to the story).
Now, do we need two tags to differentiate that? Are people aware of that when asking questions? Does it really matter? I don't think so.
John, I think you differentiate between them very well, and they are not at all the same thing.
Planning is before you start writing, and could cover questions like "What kinds of techniques do you use to create a character?" or "How much work do you have to do on a character if you're a pants writer?"
Development is as the story is unfolding, and covers things like "What does my character have to suffer to bring him from stone-faced killer to sobbing over a kitten with a cold?" and "My sheltered nun is turning into a raging vengeance machine. How do I make this believable?"
Keep the tags separate.