How do I duplicate a whole line in Vim in a similar way to Ctrl+D in IntelliJ IDEA/ Resharper or Ctrl+Alt+↑/↓ in Eclipse?
yy or Y to copy the line (mnemonic: yank)
dd to delete the line (Vim copies what you deleted into a clipboard-like “register”, like a cut operation)
p to paste the copied or deleted text after the current line
P to paste the copied or deleted text before the current line