They're generally unique within a country but not between countries. Eg "Intrepid Mines trades in Canada on the TSX as IAU whereas in the US, IAU is the iShares gold trust ETF" [0]

[0] https://www.quora.com/Can-2-publicly-traded-companies-have-t...

I don't think Symbols are technically unique between exchanges, since some exchanges are very old, although I would have trouble coming up with an example where the major US exchanges have conflicts. Given that the Google Finance has a search bar prominently on the page. I would expect/hope that the top result would be used in the event of a conflict. It's very probably in the event of a conflict that one of the potential companies is very common while the rest are niche listings on foreign exchanges anyway.

It's a horrible mess. As someone mentioned, some use numeric symbols that I'd expect not to be unique, and while I assume most are unique because everyone uses a different format, you will probably have collisions.

Even worse is that there seem to be variants (or is it the same stock simply traded on different marketplaces), which may be denominated in the same or a different currency.

And getting from one of the symbols to an ISIN and from an ISIN to the symbol is rarely easy. And I'm not sure if an exchange:ticker tuple always maps to exactly one ISIN (and one ISIN can definitely map to multiple exchange:ticket pairs).

at a point in time and within a country/region, mostly yes. It works fine for humans, but is really messy when you try to make it rigorous enough for software.

Something as simple as Wikipedia disambiguation pages would work...