Remove duplicate errors from `dab validate`

When i run the cli using the config file created in this test, i see duplicates of the errors Aaron mentions. While I understand that the errors may be sourced from different places, this makes the errors pretty noisy if there are duplicates..
- "Table Definition for Book has not been inferred" -> appears twice

_Originally posted by @seantleonard in https://github.com/Azure/data-api-builder/pull/2100#discussion_r1535912828_

Start by reproducing `dab validate` with the config file created in the referenced test, then inspect where the reported validation errors are collected from their different sources. The work is done when duplicate messages such as “Table Definition for Book has not been inferred” appear only once while distinct errors remain visible.

Written by the indexing model from the issue text.