We should check for duplicate ping definitions as well as duplicate metric definitions

[chutten]

At present we check for duplicate metric definitions, but we don't check for duplicate ping definitions.

We kinda-sorta have schema generator keeping an eye on this because trying to create two tables with the same names but different columns will bork pretty hard, but catching things here would catch things earlier. And it's considered best practice to catch things earlier (not leastwise because we'll have better context to make better error messages).