Field API names can't be duplicated because Salesforce doesn't include the field namespace when evaluating dynamic query logic. These field names have to be re-named in order to prevent errors.
1. Go to Setup in Lightning.
2. Search for the label of the field in the top Search Setup bar (i.e. "distributor discount");
3. Open all of the results and compare which field API names are the same as the managed field API names (without the "SBQQ__" prefix).
4. Rename the custom fields to have a different API name than the SBQQ field. For example:
Object: Quote Line
Field: Distributor (Distributor__c)
Distributor__c changed to Distributor2__c will prevent the error from occurring.
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