what's the difference between 己 and 已

己:self jǐ

已 :already yǐ

自己：uno mismo
已经： ya

One is with open gap, the other is half open gap

similar but not the same

refer to the stroke order on trainchinese.com to see the difference. These slight differences can be found in many characters and are very important regarding pronounciation and meaning

已經already
自己myself（沒封閉）

they do look familiar, but definetly not the same. the first one is ji, mostly you can see this character in a word 自己，and character 己 mostly gonna be related to oneself. the most usual use of 已yi is gonna be 已经 i guess? and most of the words with this character in it gonna mean that action or something else were finished.