Fixed the filtering of duplicate columns

[grzesiek2010]

Closes #6537

Why is this the best possible solution? Were any other approaches considered?

Filtering out duplicate columns that we recently introduced worked well the first time, when no columns had been generated from properties. However, during subsequent saves, the comparison with already existing columns didn't work as expected.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

This pull request should resolve the issue. I don't see any risks that need to be highlighted.

Do we need any specific form for testing your changes? If so, please attach one.

No.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

No.

Before submitting this PR, please make sure you have:

• added or modified tests for any new or changed behavior
• run ./gradlew connectedAndroidTest (or ./gradlew testLab) and confirmed all checks still pass
• added a comment above any new strings describing it for translators
• added any new strings with date formatting to DateFormatsTest
• verified that any code or assets from external sources are properly credited in comments and/or in the about file.
• verified that any new UI elements use theme colors. UI Components Style guidelines

[WKobus]

@grzesiek2010 We currently hold on testing this, as issue that is fixed by this PR still occurs on newest master version b0fd74c

[seadowg]

@getodk/testers removing "needs testing" from this as #6543 replaces it