My id boys are now 6. They look quite different but they are still used to lots of people calling them the wrong name.
But to the point - we were on holiday a week or so ago and there was a full length mirror on the wall - but it didn't look like a mirror. DT1 looked in it (without realising it was a mirror) and asked what DT2 was doing in that room. He thought his reflection was his twin (obviously a bit slow on the uptake because they were wearing different clothes ).
It made me realise - we don't have a full length mirror at home - and they only ever get to see their faces in the mirror - never their whole body.