Unfortunately, there's no logging of this information that's visible via the main interface. While you could probably get this information from a custom plugin or digging through the logs, I'd suggest that the easiest way is to include yourself in the sharing. That way, you'll get an email about the share action. In that email it tells the recipient who else received that notification. See below for how it looks for me:
Hi my Community friends! For those who don't know me, I'm a product marketer on the Confluence Cloud team - nice to meet you! For those of you who do, you know that I've been all up in your Co...
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