By default, Trunk only links to one unique SKU per sales channel and will skip linking any other duplicate SKUs. The default behavior is usually what most businesses need since any duplicate SKUs are probably due to an error that need to be fixed–Trunk automatically sends a notification if it finds any duplicate SKUs. However, if you actually need Trunk to link to all SKUs that match within the same sales channel, then you can enable this feature.
By the way, you don't need to enable this just to sync SKUs that match across different sales channels!
For an example on why you would need to sync duplicate SKUs, you might sell a unisex shoe and have it listed on Etsy as both a men's shoe size 6 and a women's shoe size 12 but want to keep stock synced to each since they are the same item. You can do this by giving them both the same SKU and enabling duplicate SKU syncing so that Trunk will link both of these together. Another example would be creating multiple listings for the same item but under different titles/descriptions for marketing and SEO purposes.
You can enable this setting within each sales channel by scrolling to the bottom and clicking on Advanced to reveal the option.
Enabling this setting will begin linking your duplicate SKUs right away.
Now when multiple listings with the same SKU are linked to the same product in your inventory, you'll see parentheses like this:
where the (4) would mean there are 4 different listings from the same sales channel linked to this, each with the same quantity of 744.
If the 4 listings were to each have different quantities, then you'll see something like this:
where the + (plus) would indicate that the lowest quantity out of all the listings is 744