We recently introduced a new feature that checks and prevents more than 1 syncing / tracking method from being active at the same time. You receive the following error when trying to connect to Fitbit - for instance - when Google Fit is already active:

You are already connected to another tracker. Please disconnect first and try again

This change prevents duplicate distance from being automatically synced to your challenge account. If you swap primary tracking devices, you can simply disconnect or turn off your current connected app before connecting to your new one.