I found some characters, that look extremely similar to each other, like
士 shi4 scholar/warrior/knight
土 tu3 earth/dust
于 yu2 (surname), in/at/to/from/by/than/out of
千 qian1 thousand, a swing
干 gan1/gan4 dry/to concern/shield, to work/to do/to manage, manage/stem
Is this a real language phenomenon? What is the name of it? Can I get a list of such "homonim sets"?