Looking at the list of Kangxi radicals over at Wikipedia, I just noticed something I haven't noticed before. Radical number 34 (夂) and 35 (夊) are virtually identical. I write these radicals in the exact same way and I've asked one native speaker who also says she writes them the same way. The example characters are 夆 and 夏.
Since I mostly use radicals to learn/teach mnemonics, I realise that the difference between these two is irrelevant. They mean (almost) the same thing and can therefore be used as one for mnemonic purposes.
However, I'm still curious about these two radicals. Is there a difference in how they are written? Is there a difference in meaning? And finally, how come that there are two very similar radicals which also has very similar meaning?