I have doubts that a duplicate ID should be an error in any case. Yes, duplicate IDs on a single web page are not ideal and usually unintended. But the block visibility settings do not necessarily allow two blocks with the same HTML ID to be shown on the same page. For example, a site may have one version of a block shown on the home page, and a slightly different version shown on every other page. Those blocks should be allowed to have the same ID, if it makes sense to the developer. I would prefer to be allowed to set duplicate IDs, with a warning.
Thanks for a very useful module that I use on every site!
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