Redundant line in Python doc popup

How to reproduce: In .vimrc, put set completeopt-=preview and set completeopt+=popup. Then open a Python file and type something, for example prin. Press Ctrl-Space to show the completion menu. Press Ctrl-N to select print. A popup will appear.

The bug: As opposed to what would have happened in a preview scratch buffer, the popup will display the function signature twice (see screenshot).