Refactor declaration uniqueness check

[ekarademir]

Discussion 🗣

After an internal discussion, the fix in #794 which was fixing #785 was more naive than intended. For the sad path that if very long names with long over laps are used for declarations, and if there is a repeating declaration name in the list, then creating the validation error would be another O(n^2) loop.

We should refactor the code to run more efficiently.

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