Clarify whether `maxRefScriptSize` takes duplicate scripts or not into account

We need to:
- add an Imp test case that showcases the difference between the implementation and specification
- change the specification to match what the implementation currently does.

Locate maxRefScriptSize in the Imp implementation and specification, then compare how duplicate scripts are counted. Add the requested Imp test case showing the difference, and update the specification to match the current implementation. Confirm the test passes and the specification reflects that behavior.

Written by the indexing model from the issue text.