For your information. It is possible to have this accidental double charging of guns using 'The Rule of Two Guage/
Bore Difference'.
To explain, as we all know a 20 will lodge in a 12.
A 12 can lodge in an 8 bore.
A 16 in a 10.
A 28 in a 16.
So the real rule to remember is do not mix cartridges ever. If various types are available in a given area pay particular attention to keeping them segregated.