I am applying for a UK visit visa for myself and my 6 year old. Can I submit 1 set of supporting documents or will we need to duplicate this for both applications?
You don't need to supply the multiple copies. This is clearly stated in the Supporting Documents Guide Section 4.
If you are applying as a family/group you do not need to provide multiple copies of the same documents
The important this is that you provide the GWF of yourself & your 6-year-old in the remarks section in both the applications.