PF: A Postdoctoral Fellow implementing a Postdoctoral Fellowship has won another grant covering reimbursement of travel and accommodation costs for conducting further research experiments in an organisation that is a partner in the ongoing MSCA project. Is this allowed, or is it considered double funding?

If they receive additional non-MSCA funding, and they do not double-fund the same project activities, then it may be fine but they should ask for the Project Officer’s permission.