"double up（ie..strike over each other : ）" V(^e^)V nor skip ___/

These followings may involve type-bar troubles caused from clashing or so..
Or only from carriage trouble..?

double up（ie..strike over each other : ）

WRITING-MACHINES --- THEIR USE AND ABUSE, GENERAL REMARKS

It is only the invisible oil that lubricates and the surplus merely catches dust and clogs and retareis the action of the machinery causing the machine to run unnecessarily hard and the letters to "double up" or strike over each other giving rise to the belief that the tensions need altering

Barron "double up"

Remington of Wyckoff, Seamans & Benedict says "it does not double up nor skip"

HathiTrust Digital Library - Out West magazine v.12 1899-1900