If you have forgotten to install a double joist in the context of composite decking, there are several options available to you.

1. Install a sister joist: If the existing joist is still in good condition, you can install a sister joist next to it. A sister joist is a second joist that is attached to the existing joist, which can help to provide additional support for the decking board.
2. Add blocking: If you cannot install a sister joist, you can add blocking between the existing joists. Blocking is a piece of wood that is installed between the joists, which can help to prevent the decking board from sagging or bending.
3. Replace the board: If the decking board is already installed, you may need to remove it and install a new board with the correct support. While this may be more time-consuming, it is important to ensure that the deck is structurally sound and safe.