To duplicate lines I use
Ctrl + D
Is there a similar way to select/copy current line using keyboard in IntelliJ IDEA?
If you go to the specific line rather at the start or positioned at a specific row and column you can use the CTRL + W to select portions of the text depending on its complexity and language.
If you are at the start of the line and you want to select the entire text it's going to require to use CTRL + W 2 times, firstly it's going to select the first character and then the entire row till the end.