Add checks to prevent the same `Conditional` from being used multiple times

[mkorbel1]

Motivation

If someone uses the same Conditional object multiple times in the same _Always or in different _Always blocks, it can cause unexpected behavior and simulation/synthesis mismatches.

Desired solution

There should be an explicit check that Conditionals are not reused. Perhaps a build-time check where each Conditional is assigned a parent _Always, and the _Always confirms there are no duplicates within itself?