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is there a API with which I can check whether a value exists more than one time in the sublist field on line items?

No, you need to iterate the lines.
There is an api call you can make to find a value but if it's not unique across the lines you'll have no idea that's the case as far as I know. It will just return a line index.

```let lineNo = transRec.findSublistLineWithValue({
          sublistId: 'item',
          fieldId: 'lineuniquekey',
          value: lineUniqueKey
        });```

It would be helpful if it at least returned an array of matches if found on more than one line.

It would been simpler to identify common values in sublist

you could use NFT then all the power of `lodash` is available.