I would assign different "roles/positions" for the students in the groups so that they each have to submit assignments seperately and as a whole to keep the group on track.

For group projects where one student is clearly doing most of the work, even though they’re denying it, it’s important to step in and support the group dynamics. You could try having a check-in conversation with the group as a whole to get their perspective on the workload and ensure everyone is contributing fairly. You might also ask the student taking on most of the work to share their progress and how they’re managing the project with their group members. This can help them articulate their contributions and clarify roles and responsibilities. It’s also helpful to remind the group that their final grade will be based on how well they worked together, so they need to hold each other accountable for their share of the work.