Mystery error when APPENDing query: “The field 'Field4' already exists in the record.”

[ghpbi]

Hello all,
I’m stuck on an error/circumstance I’ve not run into before. When attempting to APPEND Query2 to Query1 . . .

. . . an error is generated: “Error: Expression.Error: The field 'Field4' already exists in the record.”

However, between the two queries involved, ‘Field4’ only exists one time. It doesn’t exist at all in Query1. And in Query2 it only exists once.

Notes:
1. All of the search results I’ve located that contain resolutions for the error refer to legitimate instances of duplicate field names. But that’s not the case here. Also, recall this is an APPEND, not a MERGE.

2. The error is not consistent. For example, ‘Field5’ in Query2 does not create an error during the APPEND.

3. The error does not appear to be linked to a specific data Type.

4. Another inconsistency, the error occurs in some queries containing an APPEND, but not in others.

5. The issue is happening both in dataflows (Power Query Online) and Power BI Desktop (Power Query).

My next test is splitting the dataflow into individual dataflows containing only one APPEND or MERGE per dataflow.

What have I left out that you would like answered?

Please let me know your thoughts. I’m hoping someone has triumphed over this issue already. 🙂

Thank you!

[JonasArnout2]

Thx, very strange error indeed.

We encountered this error after moving some queries to dataflows, which is a non foldable source... but then again all appended tables are sourced from dataflow

[ghpbi · Accepted solution]

We found a solution for the error. The answer is wrapping offending APPEND queries with Table.StopFolding().
Table.StopFolding - PowerQuery M | Microsoft Learn