So what about the recurring Sandile? They already had Oshawott follow Ash around before he caught it, having it happen again is too much of a cheap cop-out plotwise, even for the Pokemon writers. Besides, there are plenty of times members of Ash's team have had counterparts of their evolutionary families appear- Ash's Gligar and Paul's Gliscor, his Torterra and Paul's, his Bulbasaur/Squirtle and May's, his Cyndaquil/Quilava and Dawn's...
...anyway, there's a lot of other ways Ash could get a Sandile than just "catching what's following him," and seeing what the writers have done with Team Rocket this season I have high hopes for them being able to do something more unique.
"Ruffian" could describe Dark-type Pokemon of any species pretty well, and let's not forget that the BW020 art shows Sandile clinging to Ash's leg, which seems like something a young child would do with its parent, and certainly contrary to the tough, reckless one we've seen so far. (incidentally, has the title been totally confirmed? It wouldn't be the first time we took fake names as real...)
I think that the sunglasses Sandile has more to come in the plot, and that it's related to the one Ash will get in some way. I just don't think they're the same one, and it's an awesome enough species to get away with two members of it in the series.