Uploading data in the Webmanager or via Dropbox will fail if there is a duplicate ItemID in the file to provide.
Make sure to read the complete article as large ItemIDs may need to be checked differently
Let's assume that the actual product data starts in A2, where column A is "ItemID" in the Excel sheet.
In an adjacent column, include a formula like this: =COUNTIF(A:A,A2) Then drag it down.
When the formula is dragged down, it will count the quantity of instances of each ItemID and will also check that there are not leading or trailing spaces which might mess up that comparison.
For example: 'Green ' and 'Green' and 'Green ' are all different but will import as 'Green'
You can use Data>Filter>AutoFilter on the formula column to make sure that there should not be more than one instance for any ItemID. If you see duplicates, you can resolve them by removing the duplicates manually. But what if there are a ton duplicates?
How do you find duplicates when an ItemID can be in multiple categories and you get a duplicates error in a product Categories file upload?
In Excel, combine the ItemID and the Categories column in an adjacent Column
This example has a formula in D2 that also removes spaces from the ItemID and the Categories field and combines them: =TRIM(A2)&TRIM(B2) Drag that down in Column D.
Column E has the same COUNTIF formula that was used previously, but it is adjusted to look at the content in D2 and dragged down. It does not need the TRIM function in it, because we have already removed all spaces in A and B in the formula found in D2.
This formula in E2 will be =COUNTIF(D:D,D2) and then dragged down
Again, look for any number larger than 1 and you will have found a duplication in the combination of the ItemID and Categories.
IF you have ItemIDs larger than 15 characters..
If your ItemIDs are longer than 15 characters long then Countif can not successfully compare.
You should then instead use the SUMPRODUCT function a formula such as =SUMPRODUCT(--(A$2:A$24810 =A2))