Little very useful tip i think is underused.
Type 【Alt+/】 to expand a word that is already in the buffer. Type that again to switch to the next possibility.
Suppose your are coding in python. You have defined your own function “getStringInFile”. Now, when you need to type that again, just type “ge” then press 【Alt+/】. If the word is not you want, press that again.
You can see all available choices by pressing 【Ctrl+Alt+/】.
I used emacs daily since 1998, but didn't learn this command until about 2008.