But it's not a strange decision at all (and certainly not top 5 in a list of key moments).
Firstly, VLP wasn't past him, they were close together, shoulder to shoulder in what could have been argued (if you're not aware of VLP pulling Duffy in) a "smart" defensive challenge - the sort of...
But there were a lot of times the ref gave a free kick and not a yellow, the only possible difference is that in the Duffy incident he shouldn't have given the free kick. If it shouldn't have been a yellow, the ref not giving the yellow isn't significant.
If it's the same incident (was out wide on the east side south end), even if it's obstruction, Van La Parra had overhit the ball and was never going to collect it before Ryan, so the free kick would have been enough. Though as I say, replays show it was Van La Parra pulling Duffy into him, so...
Why should Duffy have been sent off? The only thing I remember that might be a consideration was when he seemed to block of Van La Parra, but replays showed Van La Parra was pulling Duffy's arm and pulling Duffy into him, so it shouldn't have even been a free kick. Am I forgetting another incident?