Guides

Same SKU on several products: how to share one inventory count

Shopify keeps a separate count for every product, even when two carry the same SKU, so the same item sold as two products is two stocks that go out of step with the first sale. There are two ways to make them share one count: merge them into one product with variants, or link them by SKU with an app such as StockUnison.

Shopify cannot share one count between products on its own. Every product variant keeps its own stock, and the SKU field is a label that links nothing, so two products carrying the same SKU are two separate counts. There are two ways to make them one:

1. Merge them into one product with variants, if they really are one product. This needs no app.
2. Link them by SKU with an app, when they need to stay separate products: a mug sold on its own and inside a gift set, or the same scarf in two collections under two names. StockUnison’s shared stock does this. It finds every product carrying the same SKU, shows you what would change before anything is written, and from then on a sale, restock or edit on any of them changes all of them within seconds. It is free up to 50 SKUs, and $9 a month for up to 10,000 SKUs in one store.

Setting it up in StockUnison

1. Install StockUnison from the Shopify App Store and open it in your admin.
2. Press “Share this store’s stock” and pick the location whose stock is shared. With one location, it is chosen for you.
3. Press “Check my catalogue.” It reads every product and lists each SKU that is on more than one product, with the count each product holds now. Nothing changes yet, and you can check as often as you like on the free plan.
4. Review the list, then go live. Going live sets the products of each SKU to their lowest count, and from then on they move together.

The help page has the same steps with every option, including sharing from several locations and adding more stores.

Why Shopify counts them separately

Shopify’s inventory model has one assumption in it that catches people out: a product variant owns its stock. Not the SKU — the variant. The SKU is a text field you type in, it is not required to be unique, nothing validates it, and nothing in the admin treats two variants carrying the same one as related.

So the moment the same physical item exists as more than one product, you have more than one stock, and they go out of step with the first sale.

The four ways this happens

None of these are mistakes. They are all reasonable things to do, and Shopify offers no warning that any of them splits your stock.

• Bundles and sets. A mug sold on its own, and the same mug inside a gift set. Ten mugs on the shelf; Shopify believes there are ten of each product.
• A duplicate made for a campaign. Someone duplicates a product for a sale, a landing page or a wholesale price, and it never gets merged back. Both stay live.
• The same item under two names. “Navy wool scarf” in the main collection and “The Harbour scarf” in a curated one, because collections could not do what the merchandiser wanted.
• A second store. The most visible version, and the one most apps are built for — but structurally identical to the three above.

The symptom is always the same, and it is always late: you sell the last one twice, and find out from the customer.

What doesn’t fix it

Variants, when the products must stay separate. If the two are genuinely one product, merging them into one product with variants is the correct answer and you should do that instead of installing anything. It stops being the answer the moment the two need to be separate — different titles, different collections, different pricing, a bundle that contains the item rather than being it.

Shopify’s own multi-location inventory. This tracks one item across many places, which is a different axis entirely. It will not help two products in one location.

A spreadsheet and a calendar reminder. It works until the day it matters, which is the day you are busy. The counts move apart on every sale, not on a schedule.

Setting one of them to “continue selling when out of stock”. This does not share anything. It hides the symptom by letting the oversell happen silently.

What does: one count per SKU

The fix is to stop treating the SKU as a label and start treating it as the key it was always meant to be. Every product carrying the same SKU sells from one count, and a sale on any of them lowers all of them.

Who does it

Fewer apps than you would expect, and the public record in this particular niche is the weakest in the whole category. Every figure read from each App Store listing on 18 September 2026, Duplicup’s on 27 September; “≤3★” is the share of reviews at three stars or below, from each listing’s own rating breakdown.

Two of those complaint shares are the highest we have measured anywhere — 24.5% and 43.8%, against a store-to-store tier that runs between 0% and 18.3%. Small bases move fast and sixteen reviews is thin evidence, so read them as a warning to check rather than a verdict. But it is a fair summary of the niche: the multi-store side of this category has apps with excellent records, and this side does not. Trunk is the exception, and if any of your duplicate products sit on Etsy or Amazon it is the answer rather than us.

One more app belongs in this decision and is not in the table above, because it arrived at this job from the multi-store side rather than starting here: Easify Inventory Sync, whose headline claim is duplicate SKUs, bundles and raw materials sharing one count inside a single store. Read from its App Store listing on 20 September 2026 it carries 72 reviews at 4.5 with 11.1% at three stars or below — a higher complaint share than Inventory Sync ‑ GoGo’s, on a base a third larger — and it is the only app doing this job that holds Shopify’s Built for Shopify badge. Worth weighing before the smaller specialists.

The newest app in the table is Duplicup, launched in May 2026 and built for exactly this job. One thing sets it apart from the rest of the table, us included: it can link products whose SKUs differ. Its four reviews are all five stars and all about fast, personal support, which is early evidence rather than a record. The thing to model is its meter: it prices by SKU count, free for 3, then $19.99 for 50, $49.99 for 250 and $129.99 for unlimited, so a catalogue with a few hundred shared SKUs lands on the top tier.

How ours works

StockUnison’s shared stock reads your catalogue and finds every SKU that is on more than one product — inside one store, or across as many stores as join with your code. From then on each of those SKUs holds one count, and every product carrying it shows that count.

Four things about it are worth knowing before you turn it on.

1. You see it first. The check reads your catalogue and shows exactly which products share each SKU and what each one currently holds. Nothing is written until you go live.
2. Going live sets each SKU’s products to their lowest count. If the mug reads 10 on its own and 4 in the set, both become 4. The reasoning is one-directional: showing more stock than you have is what causes overselling, so the safe move is always down.
3. A SKU on one product is left alone. There is nothing to share, so nothing is touched.
4. It stops rather than guesses. If the numbers stop adding up — a CSV import, another app writing the same stock — StockUnison stops updating that SKU and asks you to look, instead of spreading a count it cannot explain. That matters most here: the more products share a SKU, the more times a second writer’s change gets counted.

Getting your SKUs right first

Shared stock is only as good as the SKUs, and this is the part worth doing carefully:

• A SKU identifies a physical thing. If two products carry the same SKU but are not the same item on the shelf, sharing their count is wrong and will cause the oversell rather than prevent it. Fix the SKU first.
• Case and whitespace are normalised, so MUG-01 and mug-01 are the same SKU. This is deliberate — two spellings of one code would otherwise be two counts, which is exactly the problem this feature exists to remove.
• Products with no SKU are skipped entirely. They cannot be matched to anything, and guessing from titles is how stock reaches the wrong product.
• If you have inherited a catalogue where the same code means different things in different places, sort that out before turning anything on. The free duplicate SKU checker lists every SKU on more than one product from your product export, in your browser, and the duplicate SKU guide covers what to do about each.

When one store in charge is the better shape

Shared stock is symmetric — every product carrying the SKU is equal, and a sale anywhere moves all of them. That is right when they are genuinely one shelf.

It is the wrong shape when one store’s number is the real one and the others should merely reflect it: a supplier feeding resellers, or a retail store feeding a B2B store that should never write back. For that, put one store in charge instead, which copies one way and never lets a connected store change the main one.

The test is a single question. If a sale in any of these places should lower the stock in all of them, share one stock. If only one place’s count should count, put that one in charge.

What it costs

Checking is free and unlimited — install, let it read the catalogue, and look at what it found for as long as you like, at any catalogue size. Going live is free up to 50 SKUs, and $9/month for one store with up to 10,000 SKUs sharing; the pricing page has the tiers, and only one store ever pays however many join.