Repeat connection between two devices

Right now its easily possible to connect two devices more than one time. But it probably shouldn't be possible to connect more than once (if both devices know each other).

Start by tracing the mobile app's device-connection flow and how it checks whether two devices already know each other. Reproduce a repeated connection, identify the relevant entry point and tests, and confirm that a second connection between the same devices is rejected or ignored.

Written by the indexing model from the issue text.