identify & warn for identical periodic columns

Comment for the future: what should we do if two periodic columns are identical (same cycle length and values)? Should it be an error? A warning? Or something else?

_Originally posted by @bobbinth in https://github.com/0xPolygonMiden/air-dsl/pull/24#discussion_r1003624902_

This should probably be handled as a warning.

We could also consider doing an optimization to identify duplicate periodic columns and always reference the one that was declared first instead, then remove the duplicate from the generated Air. This may be more work than it's worth though, and is lower priority than issuing a warning.