It's more like the plural of 'deer' being 'deer'. I don't think I've ever heard anyone attach an 's' to 'kanji' for pluralization when speaking English. Like how it might be odd to say 'sushis' or 'wasabis'. Whenever I need to stress plurality i would say 'kanji characters' or something like that. Oxford English Dictionary lists 'sushi', 'kanji', 'shinkansen', 'katakana' all as being mass nouns or having the plural form the same as the singular. The exception in the words I looked up was 'tsunami' which may be pluralized as 'tsunami' or 'tsunamis'.