I've noticed a few kanji that appear identical to a kana. 二 is the only example I can remember, where it represents both the kanji ni and the katakana ni, although I'm sure I've seen others. Are there many examples of this? Are they coincidental?
I'm no expert in the history of the Japanese writing system so I'm going to be putting a lot of faith this chart and the idea in general that katakana are derived from small parts of larger kanji. This appears to be generally accepted though Japanese wikipedia notes opposition by one scholar.
If we go by this chart, it's no coincidence that katakana ニ looks like the kanji 二, since it was taken from the kanji 仁 which itself is made up of 人 and 二. Similarly, カ is taken from the 力 in 加, エ is taken from the 工 in 江, ロ is from the 口 in 呂 and チ is directly from 千. I think that covers all ones which are very similar visually.