The click is redirected to the product on the merchant's domain, so it is the same merchant with the same domain name, even if the product data feeds are managed in different Google Merchant Center accounts.
Google does its utmost to ensure that the same product is never displayed multiple times. One of these measures is to look at multiple parameters in the product feed, including MPN, GTIN, brand, EAN, etc. When Google detects a possible duplication, it usually displays another relevant product from the same ad group instead.
When merchants use a multi-CSS campaign (e.g. CSS 1: through Google Shopping + CSS 2: through Connexity) the bids for a given product by merchant 1/CSS 1 are actually determined by the next best bid of another merchant, i.e. merchant 2/CSS 2, since Google runs a secondary price auction.