A TODO in [GrammarConsistencyChecker.java](https://github.com/melt-umn/copper/blob/2d4c34b32c61e3bfc88913e5245aedffcf2de523/src/edu.umn.cs.melt.copper.compiletime/src/main/java/edu/umn/cs/melt/copper/compiletime/spec/grammarbeans/visitors/GrammarConsistencyChecker.java#L360) suggests that Copper makes no check for duplicate productions.