One day in 2017 when I was out cross-country skiing with a friend I noticed we were having double shadows. In the first picture you can clearly see that my friends left ski pole has a double shadow, but this double shadow phenomenon progressively gets less noticeable further to the right. In fact, my right ski pole has no double shadow at all. Moments later, I also appeared as a double shadow while still standing at the exact same spot as in the first photo, as can be seen in the second photo. In the third picture you can see what it looked like behind us, with the sun shining through tree tops at the other side of the marsh. My guess is that one of the tree tops managed to "split" the suns disk in two parts, creating two different light sources. After all, only objects in the partial shadow of the trees gave double shadows, but not objects in direct sunlight, like the tree above my shadow in the second photo. It would also explain why the double shadow effect would move in time, as the sun itself also moves in time. Since then, I have experienced this effect many more times when skiing, every time confirming the above explanation.
Double shadows around my friend, but not around me (2017-12-30)
Moments later, my shadow was also double (2017-12-30)
What it looked like behind us (2017-12-30)
[ to the left of the tree shadow, broad head shadow || in the middle of the tree shadow, narrow head shadow || to the right of the tree shadow, broad head shadow ] ⇛ In this case, the splitting of the sun to two separate light sources also causes my shadow to be narrower when my shadow is in the middle of the trees' shadow (2020-03-07)
[ inside the partial tree shadow, double head shadow || outside of the partial tree shadow, single head shadow ] ⇛ Again a double shadow of my head (2020-03-07)