Can multiple ponies (especially unicorns) have the same talent? Could another unicorn have Twilight's talent for magic, for example?
Yes, to an extent. Ponies can have very similar talents, with very similar cutie marks. However, since everypony is unique in some way, so too are their talents, and therefore cutie marks.
One example we see of two ponies sharing similar talents are Flim and Flam.
They both share a similar talent for
scamming other ponies out of their bits salesmanship, and their cutie marks are very similar, complementary even. You'll notice though that their cutie marks are not identical, despite their very similar talents.
One more recent, and perhaps even better example of ponies having similar talents (and similar Cutie Marks) which we see in a later episode of season 5 is
the Cutie Mark Crusaders.
These ponies share a similar talent of
helping other ponies discover their special talents.
and again, we see this reflected in their similar cutie marks:
Notice again though that while these ponies' cutie marks are similar, they also have some differences that reflect the ways in which their talents are somewhat different.
There are numerous other examples of this in the series, with ponies who have similar talents, but not identical ones. Both the Apple family, and the flower ponies (Daisy, Lily, and Rose) come to mind.
So in the example you gave of Twilight, I would say that other ponies could share a similar talent for magic (with perhaps even a similar cutie mark) as Twilight, but nopony could have the exact same talent for magic that Twilight has, and similarly, no two ponies could ever share the exact same cutie mark.
There is some evidence that two ponies can't have the same special talent, but none that completely clears this up. In particular, some ponies refer to special talents as “unique”.
- Twilight Sparkle says “After all, every pony is unique and individual.” in S4 E21.
- She also says “Everypony has unique talents and gifts” in S5 E2.
- Cheerilee says “Discovering what makes you unique isn't something that happens overnight” in S1 E12.
About Twilight Sparkle in particular, I'm quite sure that nopony shares her unique talent. Princess Celestia vouches for that in S1 E23, saying “I don't think I've ever come across a unicorn with your raw abilities.”
No pony has COMPLETELY identical cutie marks, not even the spa ponies Lotus and Aloe. One has a white flower outlined in blue, the other is a white flower outlined in pink. The crusaders have the same shield with different pictures in the center. All cutie marks have some small variation, kind of like identical twins. While very similar, or ALMOST identical none are 100% identical. Hope that helps.