If an applicant is submitting a COFUND proposal, which builds on a previously successful COFUND, would this be considered as a “very similar” proposal for the purpose of ticking the box in Part A?

If it is a continuation – yes, otherwise no. The number of the project should be given. Even if the applicant mentions that a similar proposal has already been submitted, this will be checked very carefully by REA. If after checking REA sees it is not the case, they will not consider it to be similar.