When completing a Questionnaire, if Tide detects that a header row(s) selected have columns with the same title/name (e.g. Tax Amount appears in columns X and Z), you will be required to confirm that you are happy for Tide to additionally reference each column by it's reference too (e.g. Column - A). Tide will then display all columns with their reference appended to the end (e.g. Insured Name [X]).
This then fixes the Questionnaire as using duplicate column names. During processing the columns must be presented in the same exact column position (as Tide cannot know which column of the same name should be assigned to relevant Tide field).
If these columns are not reported in the correct sequence, then the error 'Header columns are not in the correct sequence' is raised. This error lists out the columns and their expected row position. You will need to re-submit a revised Bordereau with the header rows in the correct sequence order. If the sequence order has permanently changed, or if the header row now presents uniquely named columns, then you will need to complete a new Questionnaire.
For more information on how Tide identifies headers in a Bordereau, visit the Identifying the Header in a Bordereau article.