Say Tony didn't drop the Space stone that Loki picks up in 2012, so the Avengers still have it. What would happen if they go back to the 70's anyway and see the space stone at the SHIELD base and take that one, would they then have 2 Space stones?
Ah time travel. Basic answer is, yes they would, 2 versions of the same one.
Putting the Space gem back was more awkward. The Tesseract was it's case, Thanos destroyed it in Infinity War by crushing it, so the Avengers would have had to do the same, how did they recreate it?