If it’s dark, both rear red lights will be on. It could be motorcycles, if they are really close together but that sounds unlikely.
Even in the event of failed lights, the clusters have reflective sections that will illuminate in response to your own lights.
What I failed to articulate (I can’t get my brain to boot this morning) is that a decent amount of motorcycles use arrows as their turn indicators. When I see a blinking arrow at night, I assume it’s a bike.
Just be careful, as you can see from this article it might not be the case. Better to be on the lookout and don’t assume things :)
If you're following close enough that this is an issue then you are driving too close for the conditions.