So, if this is an older Doctor travelling back on Clara's timeline some time after witnessing Clara's death, what was his younger self doing while his older self was back with Clara? Had he gone off in a sulk (or perhaps to save the Universe) after one of the (many) fall outs in series 8? Maybe after Clara tried to hijack the TARDIS? And what does the older Doctor do when it comes time for Clara's death? Does he standby and let it happen (because he's already seen it (from another viewpoint)), or does he try to save her, knowing it would create the Clara inversion of the classic grandfather paradox.
If he saves her, he'd never experience the event that gave him the reason to go back in time and save her, so he wouldn't go back in time, so she would need saving, so he would go back in time, so she wouldn't need saving, so he wouldn't go back in time, etc, etc.
After travelling without Clara for some time, the Doctor hears she is in mortal danger and decides to visit the moment of her peril (maybe he thinks then he can avert it?), but is unable to save her and is surprised to see a version of himself with Clara in her final moments. Is it a Zygon clone? How come he has a TARDIS? He decides to go further back along Clara's timeline and finds evidence he's been with her all along. He decides to contact Clara to warn her that it's a duplicate Doctor, but decides not to tell her he's from her future (for obvious reasons). He travels with Clara, waiting for the duplicate to show up. That doesn't happen. His older self was the duplicate all along, and now he faces the choice of whether or not to save Clara all over again.
I'm not saying anything like that is right, but it would explain why he's relatively mild this season as compared to last.