Tipping users on Duelbits is pretty straightforward. However, there is one limitation to it. A user who intends to tip another user must have deposited at least 100 coins.
You just need to click the desired user's name in the chat, which will bring up this dialog box:
Click the "Tip User" button, enter your desired amount to tip in the shown input, and click "Tip." Please note that the minimum amount to tip is 1.00 coins.