MixinItemRenderer targets render at renderByItem:

It calls RenderProvider.of(stack).getCustomRenderer().renderByItem(...), and nothing else.
Looking at the code, this seems to just call the vanilla BEWLR a second time:

• RenderProvider.of returns DEFAULT for a non-geo item
• DEFAULT returns MC's BEWLR
• calls renderByItem on the returned BEWLR

The original call is not canceled, so both happen.

There's two problems here:

• geo items go through custom rendering and vanilla rendering
• non-geo items rendered via BEWLR are always rendered twice

For most items, this is not an issue (besides lag), but for some custom ones, it is: