When a form receives data from multiple sources, one source may overwrite values populated by another. For example, Marketo’s Munchkin API may overwrite existing field values.
To preserve the original values, you can configure Marketo to block updates to non-empty fields. This setting can also be limited by source, allowing you to control which updates are blocked.
See Marketo's article Block Updates to a Field.
Once configured, Marketo blocks updates to existing fields that already contain values. Empty fields can still be populated when data is first submitted.