As someone who is toting around a smidgen of unnecessary weight, I make sure my sentences are sleek and streamlined.
So, let me ask you: What’s the difference between “sleek” and “streamlined”?
Answer: Not enough. (Actually, the two words are pretty much synonymous.)
So why would anyone lard up his or her prose with equivalencies? In my world, an equivalency is created when two equivalent words are joined by and or but, like so:
knowledge and expertise
valuable and precious
charities or causes
unique and special
thankful and appreciative
Sometimes, the issue is not quite so clear cut, and can be up to the author, but here, for example, I think one word, not both words, is enough:
I am involved in many worthwhile groups and activities in my town.
Take a look at your writing, and watch for equivalencies. Pick le mot juste, and go with that. It’s easier to lose a word here or a word there than to lose 20 pounds!